add support for [1974164] Be able to take a screen shot on the device

screenshots are taken using ISurfaceComposer::captureScreen() which returns
the size of the screenshot and an IMemoryHeap containing the data.
screenshots have limitations:
- they will always fail if a secure window is up on screen
- require GL_OES_framebuffer_object extension
- in some situation, video planes won't been captured

Change-Id: I741c68a2d2984fb139039301c3349e6780e2cd58