Use GraphicBuffer instead of GBP for screenshots.

Migrate screenshot code to render the layers into a
GraphicBuffer instead of creating a GraphicBufferProducer. This cleans
up the code and makes rendering a screen capture simpler and clearer.

Test: Screencaptures for Recents, manual screenshots, and "adb shell
screencap"
Test: Transaction_test

Change-Id: Ifb463c0e98cfaa3f96ad27837b1a2e2921e253d1