Runs the A11yManagerService package monitor on a dedicated thread.

The default BackgroundThread used by PackageMonitor is shared by
other components and can get busy, causing a delay and eventual
ANR when responding to broadcasts sent to this PackageMonitor.

Fix: 348138695
Test: existing tests in `atest AccessibilityManagerService`
Test: install and uninstall packages, observe no change in behavior
Flag: com.android.server.accessibility.package_monitor_dedicated_thread
Change-Id: Idd112cfeb7e3e55d18d4596c28a90450aa72f682