Return the ZIP 32 account index from RustBackend.getAccounts

Previously we were returning the internal account private key, which
is incorrect for how `Account` is currently used on the Kotlin side.

Closes #1638.

backend-lib/src/main/java/cash/z/ecc/android/sdk/internal/model/JniAccount.kt

@@ -5,19 +5,19 @@ import androidx.annotation.Keep
 /**
  * Serves as cross layer (Kotlin, Rust) communication class.
  *
- * @param account the account ID
+ * @param accountIndex the ZIP 32 account index.
  * @param ufvk The account's Unified Full Viewing Key, if any.
  * @throws IllegalArgumentException if the values are inconsistent.
  */
 @Keep
 @Suppress("LongParameterList")
 class JniAccount(
-    val accountId: Long,
+    val accountIndex: Int,
     val ufvk: String?,
 ) {
     init {
-        require(accountId >= 0) {
-            "Account ID must be non-negative"
+        require(accountIndex >= 0) {
+            "Account index must be non-negative"
         }
     }
 }

backend-lib/src/main/rust/lib.rs

@@ -297,17 +297,22 @@ fn encode_account<'a, P: Parameters>(
     network: &P,
     account: zcash_client_sqlite::wallet::Account,
 ) -> jni::errors::Result<JObject<'a>> {
+    let account_index = match account.source() {
+        AccountSource::Derived { account_index, .. } => account_index,
+        AccountSource::Imported { .. } => panic!("Should have been filtered out"),
+    };
+
     let ufvk = match account.ufvk() {
         Some(ufvk) => env.new_string(ufvk.encode(network))?.into(),
         None => JObject::null(),
     };
 
     env.new_object(
         JNI_ACCOUNT,
-        "(JLjava/lang/String;)V",
+        "(ILjava/lang/String;)V",
         &[
             // TODO: This will be replaced by the multi-seed-compatible account ID.
-            JValue::Long(i64::from(account.id().as_u32())),
+            JValue::Int(u32::from(account_index) as i32),
             (&ufvk).into(),
         ],
     )
@@ -336,6 +341,13 @@ pub extern "C" fn Java_cash_z_ecc_android_sdk_internal_jni_RustBackend_getAccoun
             })
             .collect::<Result<Vec<_>, _>>()?;
 
+        // Filter out imported accounts (for which we don't know a ZIP 32 account index).
+        // TODO: Remove this when we switch to account identifiers.
+        let accounts = accounts
+            .into_iter()
+            .filter(|account| matches!(account.source(), AccountSource::Derived { .. }))
+            .collect::<Vec<_>>();
+
         let first_account = accounts.first().cloned();
 
         Ok(utils::rust_vec_to_java(

sdk-lib/src/main/java/cash/z/ecc/android/sdk/internal/TypesafeBackendImpl.kt

@@ -27,7 +27,7 @@ internal class TypesafeBackendImpl(private val backend: Backend) : TypesafeBacke
     override val network: ZcashNetwork
         get() = ZcashNetwork.from(backend.networkId)
 
-    override suspend fun getAccounts(): List<Account> = backend.getAccounts().map { Account(it.accountId.toInt()) }
+    override suspend fun getAccounts(): List<Account> = backend.getAccounts().map { Account(it.accountIndex.toInt()) }
 
     override suspend fun createAccountAndGetSpendingKey(
         seed: ByteArray,