Revert "ANDROID: GKI: Revert "mm: unexport free_reserved_area""

This export was added to handle the cleanup of bootsplash memory,
however this problem should be solved differently in future versions of
Android, so remove the export so that drivers do not start to rely on
it.

This reverts commit 62a48a7e.

Bug: 139653858
Bug: 146846822
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: I9b381353fda446526463333a70428145b710298a