Make AppOpsUidStateTracker callbacks more testable

The unit tests will become easier to write when validating callbacks
and when having delayed behaviors. This is done by 1: using an executor
instead of a handler so that we only need to check if the execute mock
is invoked and 2: a custom executor interface that supports delayed
execution. Executors are preferred so that this interface will be more
consistent with public api guidelines and also because post/send methods
in Handler are final so it's impossible to decouple a subclass/mocked
Handler's behavior from the current Looper. Finally the test clock
implementation takes the custom executor in the constructor so it can
tell the executor when time changes so it can execute delayed runnables.

Test: AppOpsUidStateTrackerTest
Bug: 247111208

Change-Id: I930e7e56630408440f6ca03b4c2ab84b19b20eb9