Scaffolding for AnnotatedUserHandles testing.

This provides (test-only) capacities to inject different
UserHandle values than the ones we'd normally determine, so that
we can test under artificially-constructed scenarios just by
injecting the appropriate fake AnnotatedUserHandles value.

Test: `atest AnnotatedUserHandlesTest` (& presubmits/etc)
Bug: 280237072
Change-Id: Idc2f7a5a46f49f9e4c11d361e01e6943404262b2
3 files changed