Deprecate InputMonitor and monitorGestureInput

Gesture Monitors are deprecated in favor of spy windows, since they
offer a much greater degree of customizations for targeting certain
events, rather than listening to all events over the entire display.

Bug: 245989146
Change-Id: I58105c441ca5161cde07d5440c20824c14dbacef
Test: Presubmit
Flag: NONE docs only