Better guarantee a11y service initial state

I've see flakiness in the gesture dispatch test with
magnification enabled. It turned out that the state
of the input filter was in flux when onServiceConnected
was called back. Now delaying that call until the
input filter's state is stable.

Also fixing the return value of getCenterX and Y when
a service isn't actively controlling magnification.

Test: A11y CTS and unit tests.
Change-Id: I64260ebb72ee95307d777a9e4b70876c14f99e9c