Always set usage bits for GPU comp in DisplayDevice.

* Always set GRALLOC_USAGE_HW_RENDER as usage bit
* Always use RGBA8888 buffer for the GPU to render to.

Otherwise, some virtual display sinks can request a YUV buffer, which
the GPU cannot render to.

Also add back in a connect call for the scratch buffer, since that
should not have been removed.

Bug: 117680609
Change-Id: Ia0c2e99db839ff681529c041a471242d25a8f743
Test: Simulating virtual displays
Test: adb shell screenrecord
Test: Chromecast display mirroring
Test: Simulating virtual displays while mirroring displays