fix(touchexplorer): Avoid conflicts with real and TouchExplorer events by using a virtual device id

InputDispatcher's touch streams track touch state consistency separately
for each device id. Using a virtual device id for TouchExplorer, like we
already do for MotionEventInjector, ensures that these events do not
clash with real touchscreen events that happened before the input filter
was enabled.

If a real pointer is on screen when TouchExplorer starts then
InputDispatcher will automatically clear that real deviceId's pointer
state when it sees an event from the new virtual deviceId on the same
display.

(Note: device id != display id. Real and injected events target the same
display, but use different device ids to indicate who made the event.)

Bug: 364408887
Test: manually use touch exploration and observe issue in bug is gone
Test: atest com.android.server.accessibility.gestures.TouchExplorerTest
Test: atest android.accessibilityservice.cts.TouchExplorerTest
Flag: com.android.server.accessibility.touch_explorer_use_virtual_device_id
Change-Id: I223d50bf9ce53e0ff366f43d2f913c7d119b789d