L2CAP fixes for the holder.

This code adds a fallback so that in the event an exception is thrown by the
L2CAP socket during connection, it will fall back to the old flow.  This is
necessary when interacting with the iPhone reader, as Android devices do not
seem to get along with BLE link-layer encryption as implemented in iOS.

MobileSdk/src/main/java/com/spruceid/mobile/sdk/GattClient.kt

@@ -47,6 +47,10 @@ class GattClient(private var callback: GattClientCallback,
         UUID.fromString("00002902-0000-1000-8000-00805f9b34fb")
     private val L2CAP_BUFFER_SIZE = (1 shl 16) // 64K
 
+    enum class UseL2CAP { IfAvailable, Yes, No }
+
+    private var useL2CAP = UseL2CAP.IfAvailable
+
     var gattClient: BluetoothGatt? = null
 
     var characteristicState: BluetoothGattCharacteristic? = null
@@ -60,7 +64,6 @@ class GattClient(private var callback: GattClientCallback,
     private var writeIsOutstanding = false
     private var writingQueue: Queue<ByteArray> = ArrayDeque()
     private var writingQueueTotalChunks = 0
-    private var usingL2CAP = false
     private var setL2CAPNotify = false
     private var channelPSM = 0
     private var l2capSocket: BluetoothSocket? = null
@@ -429,6 +432,12 @@ class GattClient(private var callback: GattClientCallback,
                                         }
                                     } catch (e: IOException) {
                                         reportError("Error connecting to L2CAP socket: ${e.message}")
+
+                                        // Something went wrong.  Fall back to the old flow, don't try L2CAP
+                                        // again for this run.
+                                        useL2CAP = UseL2CAP.No
+                                        enableNotification(gatt, characteristicServer2Client, "Server2Client")
+
                                         return
                                     } catch (e: SecurityException) {
                                         reportError("Not authorized to connect to L2CAP socket.")
@@ -612,8 +621,9 @@ class GattClient(private var callback: GattClientCallback,
      * Log stuff both to the callback and to the console.
      */
     private fun reportLog(text: String) {
-        Log.d("[GattClient]", "ERROR: ${text}")
-        callback.onLog(text)
+        Log.d("[GattClient]", "${text}")
+
+        //callback.onLog(text) // Appears to mess with transfer timing, disabled for now.
     }
 
     /**
@@ -631,21 +641,18 @@ class GattClient(private var callback: GattClientCallback,
         try {
             // Use L2CAP if supported by GattServer and by this OS version
 
-            // L2CAP use disabled for now; the plumbing is all in place, and it ought to work, but
-            // when we call connect() on the socket we get a "no resources available" exception, and
-            // the socket remains unopen.
-
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
-                //usingL2CAP = characteristicL2CAP != null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q
-                usingL2CAP = false
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q && characteristicL2CAP != null) {
+                if (useL2CAP == UseL2CAP.IfAvailable) {
+                    useL2CAP = UseL2CAP.Yes
+                }
             } else {
-                usingL2CAP = false
+                useL2CAP = UseL2CAP.No
             }
 
-            if (usingL2CAP) {
+            if (useL2CAP == UseL2CAP.Yes) {
                 enableNotification(gatt, characteristicL2CAP, "L2CAP")
 
-                reportLog("Using L2CAP: $usingL2CAP")
+                reportLog("Using L2CAP: $useL2CAP")
 
                 //// value is returned async above in onCharacteristicRead()
 
@@ -720,7 +727,7 @@ class GattClient(private var callback: GattClientCallback,
     fun sendMessage(data: ByteArray) {
         reportLog("Sending message: $data")
 
-        if (usingL2CAP) {
+        if (useL2CAP == UseL2CAP.Yes) {
             responseData.add(data)
         } else {
 
@@ -760,7 +767,7 @@ class GattClient(private var callback: GattClientCallback,
      * When using L2CAP it doesn't support characteristics notification.
      */
     fun supportsTransportSpecificTerminationMessage(): Boolean {
-        return !usingL2CAP
+        return useL2CAP != UseL2CAP.Yes
     }
 
     /**