Add AccessibilityManager#removeClient

Whenever an app makes a Text toast, this gets routed through systemUI
starting from R. Two A11yManagerClients are registered with
A11yManagerService for every toast. Since clients aren't removed until
the process is killed, there are a bunch of clients from systemui
hanging around and potentially cause the system to crash.

We add a way here to explicitly unregister a client

Test: build and flash,
atest android.view.accessibility.AccessibilityManagerTest
Bug: 189010828

Change-Id: I3f71bac475b4539c7be5b328adeb0be35a0b5543