Expose PackageManager methods around system apps

Expose PackageManager#setSystemAppState and associated constants for use
by a future Telephony mainline module. Add restrictions on modifying
core apps if the caller is not from the system or phone uids.

Fixes: 168621241
Test: atest PackageManagerTest#testSetSystemAppHiddenUntilInstalled
Change-Id: I9a219b4b24abdb90dccca81038279bdae4285e7f