Add mocked main looper to test

Toasts were updated in frameworks/base and now will try to get
the main looper when trying to create an AccesssibilityManager. Add this
mock to avoid a null pointer when trying to post a ShadowToast.

Fixes: 151844983
Test: atest make RunSettingsRoboTests7
Change-Id: I4cb6fe44506097b52ce1bb0c8ae7a502d0901f64