Improve screenshot performance on some devices (DO NOT MERGE)

this affects devices that need a glReadPixels(). We use
a FBO instead of a GlConsumer as an intermediate render target, this
saves 2 calls to eglMakeCurrent().

On Galaxy Nexus this allows us to go from ~135ms to ~35ms for
recent's screenshots.

Bug: 8582615
Change-Id: I6b25291ecc235f1927579bbb2db3c731e985c6e8