Prefer Executor injection

Injecting a @Main Executor makes dependencies clearer and simplifies
unit tests. We don't need to deal with loopers, MainLooper annotations,
etc.

Test: atest GetMainLooperViaContextDetectorTest
Bug: 238923086
Change-Id: Iff3bce716e4b3d9903a6056e269a3a6cde5f05c1
3 files changed