staging: ion: refactor ion's heap API into linux/ion.h

Now that we know the APIs needed by system and cma heaps,
refactor and move all those into linux/ion.h.

Unfortunately, the UAPI of ion is now split into linux/ion.h
and drivers/staging/android/uapi/ion.h since some of the UAPI
leaks into heap implementation (ion flags). It can all be moved
into linux/ion.h eventually for all android kernels.

Bug: 133508579
Test: ion-unit-tests

Change-Id: I73ea59b410c46a02e23f2da5c6bdc15182a50b32
Co-developed-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Sandeep Patil <sspatil@google.com>