Edit NEM tests to prevent flakiness

- Set the Assert's main looper as the TestableLooper's looper for all
NEM tests
- in NEM test, bind content synchronously, so the callback won't run on
the 'real' main thread and will instead run on the TestableLooper's
thread
- Allow overriding already initialized dependencies in
TestableDependency

Test: atest SystemUITests
Fixes: 147685528
Change-Id: Ic171bf29e33322f44aa14c40c29c0d9ffa542ade