Separate Malloc and MemMap ArenaPools

Make ArenaPool an abstract base class and leave MallocArenaPool
implementation with it.  This enables arena_allocator to be free
of MemMap, Mutex, etc., in preparation to move the remaining collections
out of runtime/base to libartbase/base.

Bug: 22322814
Test: make -j 50 test-art-host
      build and boot

Change-Id: Ief84dcbfb749165d9bc82000c6b8f96f93052422