Refactoring A11yService and UiAutomation handling

The driver here is to make UiAutomation work across users.
I decided to take the opportunity to pull Service out of
AccessibilityService, separate the code that is service- or
UiAutomation-specific, and then manage the UiAutomation
independently from the user's state.

Bug: 63115387
Test: Adding a few tests, which is much easier after the
refactor, and running existing A11y unit and cts tests,
including those for UiAutomation. Also running the test
at ag/2593458, a sample multiuser test.

Change-Id: I2c43984d9886cd798458c9bf683c6aaa953c1afd