Refactor A11yInteractionClient so every service has its own cache

- Maintain a static collection of caches (sCaches). Lock is
sConnectionCache
- Initialize a cache when a connection is added. (client.addConnection)
- Remove the cache when a connection is removed. (client.removeConnection)
- System server should not have its cache initialized, can remove
parameter in AccessibiltyInteractionClient constructor

I tried to maintain a local reference (mAccessibilityCache) to the
service cache for each client to avoid accessing the static
collection for each request. But since the process is not immediately
killed when the service is turned off, ensuring resources are cleared
correctly is big time investment that can be done later if desired.

Bug: 192489177
Test: Build and use talkback, do the slew of tests.

atest CtsAccessibilityServiceTestCases  CtsAccessibilityTestCases
CtsUiAutomationTestCases
FrameworksServicesTests:com.android.server.accessibility
FrameworksCoreTests:com.android.internal.accessibility
FrameworksCoreTests:android.view.accessibiliity

Change-Id: Ic30f6915c7b3c186a512ed8b410bf6e842eff5fb