Allow offscreen mirrored layers to be captured.

If an offscreen layer is valid and has content, we should be able to
take a screen capture of it.

If we're mirroring content, but it's not on screen, we need
to ensure updateMirrorInfo is called to get the updated hierarchy. This
will allow the mirror content to get properly screenshot

Test: screencapture offscreen
Test: MirrorLayerTest
Test: ScreenCaptureTest
Change-Id: I31f806eb616e2d6f800da6328c9878a3e47d6a14
Merged-In: I31f806eb616e2d6f800da6328c9878a3e47d6a14
Bug: 188222480