Enable InputMonitors for non-default display.

InputFlinger today doesn't deliver InputMonitors for non-deafult
display. In order to support focussing windows on virtual-displays
(like the Vr2dDisplay), we need to be able to send motion events
to them for keyboard to gain focus.

Bug: 62033391
Test: Manual by launching
com.google.vr.vrcore/.daydream.MetaworldActivity & then
com.google.android.apps.nexuslauncher/.NexusLauncherActivity.
Exact steps are mentioned in the bug.
Change-Id: I5954e54cef8c0a29f05bc964debc95ea81f16758