Handle foreground service start failures

[xiwangly2]

#96

Improve Android foreground service lifecycle and error handling

• Add state tracking to prevent concurrent foreground service start requests and handle start timeouts.
• Wrap startForeground and startForegroundService in try-catch blocks to prevent unhandled crashes (e.g., SecurityException or RuntimeException) when starting the service fails.
• Propagate service start failures and timeouts back to Dart via the method channel result.
• Refactor disableBackgroundExecution to directly call stopService() instead of sending a shutdown intent.
• Ensure wake and Wi-Fi locks are safely released and nulled out during service cleanup.
• Update the Dart enableBackgroundExecution method to accurately reflect the native operation's success state rather than assuming it always succeeds.

Verified on Xiaomi 25060RK16C, Android 16 / targetSdk 36: mediaProjection FGS rejection is surfaced as PlatformException(ForegroundServiceStartError) instead of crashing with AndroidRuntime FATAL.