libgralloc: bring back pmem support

pmem is now legacy. ion is default, override with TARGET_USES_PMEM:=true

Change-Id: I05c95647838eb1613ddf80a9f672c2b93dc035f8

Conflicts:
	libgralloc/Android.mk
	libgralloc/alloc_controller.cpp
	libgralloc/pmem_bestfit_alloc.cpp