Fix failure in postsubmit

There are some package-pricate classes and methods in a11y framework
so we use DexmakerShareClassLoaderRule to mock them. However, this
results in some unexpected excpetions during testing. Change some of
them to public with @VisibleForTesting annotation, and remove the
DexmakerShareClassLoaderRule to avoid the exception.

Bug: 149362507
Test: atest FrameworksServicesTests:com.android.server.accessibility
Change-Id: I1e526ca79cf14b3d394ee132480dcd8c226ae69a