Cleanup PM snapshot

Bug: 181964615

Clean up some of the code that was added in support of the PM snapshot
feature. WatchableIntentResolver is renamed to WatchedIntentResolver, to
mimic the naming convention used everywhere else.

This is a non-functional change in preparation for further commits
against this bug.

Test: atest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:ScanTests
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * PackageManagerServiceBootTest
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser
 * android.appsecurity.cts.EphemeralTest
 * android.appsecurity.cts.InstantAppUserTest
 * FrameworksServicesTests:WatcherTest
 * CtsContentTestCases:IntentFilterTest

Change-Id: Ie4fc20bdcfce44adf9ec3bec8f0a36abee6db979