Fix documentation for AHardwareBuffer_(to|from)HardwareBuffer.

These functions were not showing up in the NDK documentation due to
missing Doxygen commands.

Bug: https://github.com/android-ndk/ndk/issues/706
Bug: 132738639
Test: N/A, documentation-only fix
Change-Id: I329f79da251e62b5f4f05feb7ce837c728e2cb32