Create new system API in UserHandle to get uid

For the incoming connectivity mainline, it need to compute the
intersection of a set of uids and appIds for the permission
control. As @UserIdInt values are internal implementation details
of the OS, any mainline modules should use strongly-typed
UserHandle arguments. Thus, create a new API that accepts
UserHandle and appId to get the computed uid.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: I046546d2c5c76eac7a251e63e4234776975bf2bc