Removes the a11y service from the cached service list when onPackageRemoved

The disabled a11y service is not removed from the cached service list
when the a11y service package is uninstalled.
To fix it, we have to remove the service from the service list.

Bug: 153507990
Test: manual test & adb shell dumpsys accessibility
Change-Id: Ic1d2ebe3a46faf7237123bcdfca96c79b2c35705