Make UserHandler hold a UserHandle instead of a user id

A handle gives use some more type safety. Plus, it's generally cheaper
to convert a UserHandle to the int id than viceversa.

Bug: 396394220
Test: atest CtsMediaRouterTestCases
Flag: EXEMPT refactor
Change-Id: I587ab0c93b66af9d02990bbd0d8013c299e2b641
1 file changed