Improve NDK documentation for AHardwareBuffer.

Expand existing docs with clarifications. Give descriptive names to
enums to improve Doxygen output. Move AHardwareBuffer and ANativeWindow
into separate documentation groups to reduce the mess on the
"Native Activity" documentation page.

Test: Built an image for Pixel 2.
Change-Id: I36c879dda9701d6261a8ca25ff7fb3003454939b