Add UserHandle.getAppId to SystemApi

This is consistent with other methods in this class, and unbundled
system apps such as the network stack need this API to verify their
callers regardless of the userid. The method is already marked as
TestApi.

Test: m
Bug: 123655057
Change-Id: I0ecef4abcd850dc641cd70681a56cb038778c04c
2 files changed