Use dedicated handlers in UserController

Introduced 2 handlers in UserController for messages on ActivityManager
and UiThread.

Moved UC-related parts from ActivityManager and updated unit tests.

This is a non-functional change in preparation for the next step -
introducing a lock for UserController internal state.

Test: UserControllerTest + manually switching users
Bug: 64165549
Change-Id: I26c27aeea6b1202e34f853d4b8d2cd9067f931ec