Add unit test cases to cover utility methods

UT covers the methods below:
- TelephonyPermission.isSystemOrPhone
- TelephonyPermission.isRootOrShell

The CL also introduces aconfig flag to guard the fix.

Bug: 328511085
Test: atest TelephonyPermissionsTest
Flag: com.android.internal.telephony.flags.support_phone_uid_check_for_multiuser
Change-Id: I52d638815901c8325ac49dcce13a000d47543f37