Workaround a robelectric related NPE in SurfaceSyncGroup.addTimer

Robolectric mocks out `HandlerThread` in such a way that in certain instances `HandlerThread.getLooper` returns null. Since adding the null check is small enough, we can opt to make this change to allow robolectric tests to run.

Test: atest ClockworkSettingsRobotests
Bug: 237804605
Change-Id: Ib28f80413265a770cfac44cf451714f1b66dfbc8