Expose more SystemAPI methods on AccessibilityDisplayProxy

These are:
- onProxyConnected (equivalent to onServiceConnected)
- onAccessibilityEvent
- onInterrupt
- getWindows
- set/getInstalledAndEnabledServices

Connect the proxy client (IAccessibilityServiceClient) to a
proxy service connection
(ProxyAbstractAccessibilityServiceConnection) in ProxyManager,
so the A11yDisplayProxy can access the UI.

AccessibilityServices run on their own thread. To avoid using the
app's main thread, the app that registers the A11yDisplayProxy
can call it on a different thread. Add a note in the proxy class.

Future TODOs (b/254545943):

The a11y state returned to A11yManager includes state
of all proxies and AccessibilityServices.
Separation will require AccessibiilityManager to go from being a
process singleton to a per-display instance.

Input and a11y focus are singletons and may
shift between a proxy display and a display on the device.
Separation will require future changes.

ProxyManager behaves more like UiAutomationManager,
and centralizes proxy management. Separation into different user
states will require future refactoring.

Bug: 241429275
Test: atest AccessibilityDisplayProxyTest,
AccessibilityManagerServiceTest, manual test app that
registers/unregisters A11yDisplayProxy

Change-Id: I830c6613c2c2f8abcc2343c5110f4e7b7494fb0e