Consolidate a11y unit tests and get them working

Moving tests for accessibility manager into the a11y
directory and getting them to use some of the newer
test infrastructure. Minor tweaks to
AccessibilityManager to make it testable.

Deleting the AccessibilityManagerService tests entirely.
This class is difficult to unit test with the current
infrastructure, but is covered in CTS tests to a large
degree. I wasn't able to get these working quickly, and
they rely on timeouts that make them run much slower than
the other tests. They also throw exceptions when their
mock accessibility services receive accessibility events,
which causes the system to blacklist those services, causing
the tests to fail differently until the system is reset.
I think it's better to focus on writing new, more robust
tests than on rewriting these tests to pass.

Bug: 36614219
Test: A11y unit tests now work.
Change-Id: I8effe44f22110d55ebb700dc46f59b7e7a1aa946