Provide consistent testDispatcher for Kosmos injected Settings

We want to use the same dispatcher so calls like runCurrent and advanceUntilIdle on passed testScope actually execute correctly.
This also fixes some wrong type annotations around nullability - code that should return @NonNull values now correctly returns non-nullables and Java annotations were added.

Bug: 327558308
Flag: TEST_ONLY test only changes
Test: ran deviceless tests
Change-Id: I904370a968f526102061b21515858e80b1896d1a