Convert SysUI tests to use JUnit4

Run tests with android.support.test.runner.AndroidJUnitRunner,
modification to runtest target in separate CL.

No longer inherit from TestCase, stripped unneeded code from
SysuiTestCase, which now assigns mContext via
InstrumentationRegistry.getTargetContext().

Can no longer create Handlers with default constructor,
Looper.myLooper() was never able to receive messages in tests
and it is now enforced that you pass the Looper.getMainLooper().

Change-Id: I13f499175a459cef1a554431911f4b21126e126e