ANDROID: mm/cma.c: Export symbols

Export the symbols cma_get_name, cma_alloc, cma_release
and cma_for_each_area.

These are required to modularize the ion driver.

Test: build
Bug: 147914088

Change-Id: I873fd346265be1395d2ba738aca6b9dee03700e2
Signed-off-by: Hridya Valsaraju <hridya@google.com>