Add an overload for MemMap::MapAnonymous().

Add an overload that does not take `reuse` or `use_ashmem`
parameters but uses default values (`reuse = false` and
`use_ashmem = true`) to simplify callers.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Iaa44f770dee7e043c3a1d6867dfb0416dec83b25