android: drop package attribute from AndroidManifest.xml

Fixes the following error with AGP 8.4:

    Execution failed for task ':webcrypto:processReleaseManifest'.
    > A failure occurred while executing com.android.build.gradle.tasks.ProcessLibraryManifest$ProcessLibWorkAction
       > Incorrect package="com.example.webcrypto" found in source AndroidManifest.xml: /home/simon/src/webcrypto.dart/android/src/main/AndroidManifest.xml.
         Setting the namespace via the package attribute in the source AndroidManifest.xml is no longer supported.
         Recommendation: remove package="com.example.webcrypto" from the source AndroidManifest.xml: /home/simon/src/webcrypto.dart/android/src/main/AndroidManifest.xml.

android/build.gradle

@@ -25,10 +25,7 @@ apply plugin: 'com.android.library'
 apply plugin: 'kotlin-android'
 
 android {
-    // Conditional for compatibility with AGP <4.2.
-    if (project.android.hasProperty("namespace")) {
-        namespace 'dev.google.webcrypto'
-    }
+    namespace 'dev.google.webcrypto'
 
     compileSdkVersion 31
 

android/src/main/AndroidManifest.xml

@@ -14,6 +14,5 @@
  limitations under the License.
 -->
 
-<manifest xmlns:android="http://schemas.android.com/apk/res/android"
-  package="com.example.webcrypto">
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
 </manifest>