staging: android: ion: Collapse internal header files

Ion current has ion_priv.h and ion.h as header files. ion.h was intended
to be used for public APIs but Ion never ended up really having anything
public. Combine the two headers so there is only one internal header.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>