Convert ScreenshotGraphicBuffer to ScreenshotHardwareBuffer

Bug: 150395371
Test: make && flashall

Bitmap is replacing methods that use a hidden GraphicBuffer with ones
that use the public HardwareBuffer. ScreenshotGraphicBuffer is just a
holder for GraphicBuffer and some metadata; switch it to hold a
HardwareBuffer (and rename to match).

Remove GraphicBuffer#createFromExisting, which was only called by
ScreenshotGraphicBuffer#createFromNative. Also remove JNI references to
that method, which were no longer in use.

Switch uses of GraphicBuffer that retrieve it from a
ScreenshotGraphicBuffer to HardwareBuffer.

Changes are almost entirely mechanical:
- (Screenshot)GraphicBuffer -> (Screenshot)HardwareBuffer
- GraphicBuffer#destroy -> HardwareBuffer#close
- getGraphicBuffer -> getHardwareBuffer

When creating a Snapshot in SurfaceFreezer, use the ColorSpace of the
ScreenshotHardwareBuffer.

Remove GraphicBuffer#createFromExisting. This is no longer necessary,
and although it was marked UnsupportedAppUsage, go/nonsdk-dash shows no
usage.

Change-Id: I41cb03fb65432d208820a428783de82a9f152035