Update AHardwareBuffer and ANativeWindow documentation.

This is effectively a cherry-pick of documentation improvements from
internal master. AHardwareBuffer and ANativeWindow are now in separate
documentation groups instead of being jumbled together on the giant,
impenetrable Native Activity page.

Bug: 120162384
Test: Builds and boots on Pixel 2
Merged-In: Id0b1573ebf3194b1163324da773279da95782143
Change-Id: Ib8bd403c36d792c91787258d9db0d9c1621699b1