staging: android: ashmem: Factor out compat code from uapi

Compat related code should not be exported to userspace directly.
Factor out the compat related code back into an ashmem.h file.

Change-Id: I90ae26a0045a5ff7d7af7e9caadf5d807270eaef
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>