Add ability to turn off VR virtual display functionality.

Add flag to enable/disable virtual displays via the existing
setDisplayProperties API. This makes it possible for the VR system
to turn off virtual displays if necessary after we ship.

Bug: 62546364
Test: Run 'adb shell vr enable-virtual-display [true|false] to toggle while 2d in 3d
runs.

Change-Id: Iae029be501d61189fced981dbc554e984fa7ed4b