Encapsulated access to isMainUserPermanentAdmin.

Currently, there's no API to access that info and its 2 clients
get the value directly from the framework resources, which make it
harder to "emulate" that property.

This CL encapsulates that config into
UserManagerInternal.isMainUserPermanentAdmin() and change the
existing clients to use it; a follow-up CL will change that method so
it can be emulated by a system property.

Test: atest FrameworksServicesTests:com.android.server.locksettings FrameworksServicesTests:UserManagerServiceShellCommandTest
Bug: 396232760
Flag: EXEMPT refactor

Change-Id: If894e769e986b90dce89ec7c69db95badeecfcc4