Add support for removeCallbacks in MockExecutorHandler

If the handler is constructed with a DelayableExecutor, it adds support
for cancelling runnables scheduled with postDelayed and postAtTime. It
has the same behavior as in Handler: calling it with a Runnable will
remove all pending instances of that runnable.

Test: atest MockExecutorHandlerTest
Bug: 339290820
Flag: NA

Change-Id: I47d68931ac9225efd3f1b6a211fe6475fcddaf06