Use RemoteCallbackList to store registered callbacks

Bug: 134680085
Test: Verified no errors in logcat while closing the ShareSheet
Test: Enable debug flag and verify the logs by killing the client and the servicer
Kill launcher: adb shell killall com.google.android.apps.nexuslauncher
Kill service: adb shell killall com.google.android.as

Change-Id: I706bd0f7224f24bd774ed009babc5f73e12629ca