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