[MediaProjection] Store WindowContext in VirtualDisplay

Follow on from ag/15684817, where the WindowContext is cached in
MediaProjection. To allow multiple VirtualDisplays to be created
from the same MediaProjection, instead create a new WindowContext
for each one, and associate with that VirtualDisplay.

As before, the WindowContext must be stored in a field to prevent
it from being garbage collected.

Bug: 197579637
Bug: 198415847
Test: manual
Change-Id: If3f84b3778104bf12617d224876700f8ee3b2b45