Fix AndroidManifest.xml Execution failed for task ':app:processReleaseMainManifest'.

> Manifest merger failed : android:exported needs to be explicitly specified for element <activity#net.fabiszewski.ulogger.MainActivity>. Apps targeting Android 12 and higher are required to specify an explicit value for `android:exported` when the corresponding component has an intent filter defined. See https://developer.android.com/guide/topics/manifest/activity-element#exported for details.