Add TestApi to get UserHandle from URI.

I've added this to ContentProvider since it's where all of the existing
hidden logic where userinfo = userid lives.

Bug: 136249261
Test: atest CtsProviderTestCases:android.provider.cts.DocumentsContractTest
Change-Id: I3a815f2c8dd738140aec6937ce0d3b763bf49fba
2 files changed