Clear mocks in tests

Inline mocks were leaking and the accumulation of all of them throughout
all our test led to OOM.

In order to prevent memory leak
(https://github.com/mockito/mockito/issues/1614), use suggested fix in tests
(https://github.com/mockito/mockito/wiki/What's-new-in-Mockito-2#mockito-2250).

This fix uncovered a handful of leaked Handlers. Mitigated by removing
cleaning up on teardown, but they probably should be injected in the
future so we can directly use a TestableLooper (or an executor).

Test: atest SystemUITests
Fixes: 212492048
Change-Id: I108be93088d56863aa7dae5482229b74133d3105