Update accessibility framework to support multi-session IME

Change IMMI#onSessionForAccessibilityCreated() and
IMMI#unbindAccessibilityFromCurrentClient() to take a user ID.

The main motivation is to unblock our on-going project that aims to
add concurrent multi-user support in IMMS.

All the planned use cases of A11yIME remain unchanged:
1. This CL just makes it clear that A11yIME APIs are not intended
   to be used from UiAutomator and Proxy A11yService.
2. IMMS does not use user ID yet. A subsequent CL will introduce a
   user ID verification but our plan is to not break any existing
 use cases.

This is a pure mechanical refactoring (internal IMMS refactoring),
which won’t be under any feature flag.

Bug: 305829876
Test: atest WmTests && atest FrameworksServicesTests
Test: atest CtsAccessibilityTestCases && atest CtsInputMethodTestCases && atest CtsAccessibilityServiceTestCases

Change-Id: I835e2ab07cdcbe6367698675f87d5cda160a9b5b