Fixing ControlsRequestDialogTest

ControlsRequestDialogTest was flaky because sometimes mocks were cleared (in SysuiTestCase's @AfterClass) before Activity was fully destroyed.
The solution here is to not use mock for BroadcastDispatcher and instead use FakeBroadcastDispatcher which just waits to be used.

Fixes: 214562366
Test: run ControlsRequestDialogTest 20 times locally without any failures
Change-Id: I7d7b932fe175b1e65c7ed21507b0c67a5f39dcfb