ART: More header cleanup - mem_map.h

Move MemMap maps_ into cc file and move allocator include. Remove
other unused includes. Force clients to import sys/mman.h if they
need it. Try to lower dependencies on ART by using libbase helpers
more.

Test: mmma art
Change-Id: I8f31e5f371789b3d72e12af377181fee2ac9a611