[SB][Screen Chips] Remove context.mainExecutor from tests.

I had originally added `context.mainExecutor` because I was trying to
use a real version of `DialogTransitionAnimator`, which seemed to need
that to work correctly (based on `DialogTransitionAnimatorTest`). But,
it turns out that `context.mainExecutor` makes the mock live beyond the
lifetime of the test. Once a test is done, we clear out all our mocks to
have nulls everywhere, which means that subsequent tests will see null
values when they shouldn't.

Since I'm now using a mock version of `DialogTransitionAnimator`, we
don't need these calls anyway so this CL removes them.

Fixes: 346932645
Test: atest CastToOtherDeviceChipViewModelTest
ScreenRecordChipViewModelTest ShareToAppChipViewModelTest
Flag: NONE Fixing tests

Change-Id: I248867da9856007d7539c0654e86321d4ac240a1