AccessiblityManagerService not cleaning up dead binders.

1. AccessibilityManagerService was keeping handles to dead
   IAccessibilitymanagerClients - now doing so.

2. AccessibilityManagerService was lazily cleaning up dead
   IAccessibilityServiceConnections - now using a callback.

3. Cleaned up the book keeping of enabled services.

4. Fixed a bug that the input filter is still enabled
   when disabling accessibility.

Change-Id: I5e9af7ab684a3b71e8ee51125b1262a17e960eb0