[ADI][9/N] use resource config to specify verifier package

+ Add a getter API to return the verifier package name
+ Allow a system property to override the verifier for testing
+ Shell command to retrieve the verifier package name

Note: the system property is added so that the current CTS tests can
continue to run. It will be removed after the current CTS tests are all
moved to cts-root.

FLAG: android.content.pm.verification_service

BUG: 360129657
Test: atest FrameworksMockingServicesTests_android_server_pm:com.android.server.pm.PackageInstallerServiceTest
Test: manually change the config to specify a test app which is manually
API-Coverage-Bug: 367776952

pushed onto the system partition, verify that it is specified as the
verification service provider via the shell command.

Merged-In: I25babe66665d144a45e032e005186d792455dc3b
Change-Id: I25babe66665d144a45e032e005186d792455dc3b