Add alpha channel for the EGLConfig

If the alpha channel is not assigned,the framebuffer will be Opaque.
And it will mask the underlying video and make the video invisible
when play video width mediaplayer

cherry-picked from AOSP master

Issue: b/36025630
Test: clean build and boot PixelC
Change-Id: If756ad2aaa789f6ddb178333b3e8854f52c8eac3