Filter virtual keys after touches.

Adds a new virtualKeyQuietTimeMillis configuration resource that sets
the duration for which virtual keys will be dropped after recent touches
on screen.  The default value is 0; it is intended to be overridden
per device using a resource overlay.

This change is designed to help in two cases:

1. Swipes from touchscreen into virtual key area.
2. Accidental taps in virtual key area while using on-screen keyboard.

Bug: 3089163
Change-Id: Ib912d4f8a4df9966a39cd537d3ec7c24afab7225