Fix Android crashes on sound playback and backgrounded alerts

[Copilot]

This PR fixes two critical Android crashes reported in issue #31:

Issues Fixed

1. Sound Playback Failure on Android 12+

Problem: FlutterPlatformAlert.playAlertSound() was failing with:

E/FileSource(32462): Failed to open file 'content://settings/system/notification_sound'. (No such file or directory)
E/AudioManager(32462): hasHapticChannels failure:java.io.IOException: Failed to instantiate extractor.

Root Cause: RingtoneManager.getDefaultUri(TYPE_NOTIFICATION) returns a URI that may not be accessible on Android 12+.

Fix: Added defensive programming with try-catch blocks and fallback mechanism:

• Null checks for notification URI and RingTone objects
• Fallback to RingtoneManager.getActualDefaultRingtoneUri() when default fails
• Graceful handling when all options fail (continues silently without crashing)

2. AlertDialog Crash When App is Backgrounded

Problem: FlutterPlatformAlert.showAlert() was crashing with:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
	at android.app.AlertDialog$Builder.<init>(AlertDialog.java:500)

Root Cause: AlertDialog.Builder(this.activity) was called when activity was null (app backgrounded).

Fix:

• Added null check for activity before creating AlertDialog
• Returns proper error message: "Activity is not available. Cannot show alert when app is not in foreground."
• Properly set activity = null in onDetachedFromActivity() lifecycle method

Changes Made

• Enhanced error handling in playAlertSound() method with comprehensive try-catch blocks
• Added activity null checks in both showAlert() and showCustomAlert() methods
• Fixed lifecycle management to properly handle activity detachment
• Maintained backward compatibility - API remains unchanged, just handles edge cases gracefully

Technical Details

• Modified: android/src/main/kotlin/net/zonble/flutter_platform_alert/FlutterPlatformAlertPlugin.kt
• Added 29 lines, removed 7 lines (minimal, surgical changes)
• All changes follow defensive programming principles
• No breaking changes to existing functionality

The plugin now handles Android 12+ restrictions and backgrounded app states gracefully, preventing crashes while maintaining all existing functionality.

Fixes #31.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• dl.google.com
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED -XX:MaxMetaspaceSize=512m -Xmx4g -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant -cp /usr/share/gradle-8.14.3/lib/gradle-daemon-main-8.14.3.jar -javaagent:/usr/share/gradle-8.14.3/lib/agents/gradle-instrumentation-agent-8.14.3.jar org.gradle.launcher.daemon.bootstrap.GradleDaemon 8.14.3 (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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