Add uniqueId to Virtual Display and pass through to inputflinger (1/2)

This CL adds:
1) Adds uniqueId (protected via system/sig permission) to virtual
displays.
2) Add support for N virtual display viewports into inputflinger.
3) Set the virtual display's viewports in inputflinger if it has the
uniqueId value set to non-null. (a) Moving the new viewport from java to
native inputflinger and (b) adding "uniqueId" value to viewports makes
up the great majority of this change.
4) From the inputflinger side, we also read in a new value from the
input device configuration files called 'touch.displayId'.
5) When touch.displayId and the virtual display's uniqueId match,
inputflinger links the two.

Test: Start VR and ensure that the virtual viewport shows up when running
'adb shell dump input".  Run a VR app, and ensure that the virtual input
device is associated with the new virtual viewport.
Test: com.android.server.display.DisplayManagerServiceTest

Bug: 36051620
Change-Id: Ic2117eb8e19f7f3c59687160591f8bc6692c1f12
Merged-In: Ic2117eb8e19f7f3c59687160591f8bc6692c1f12