Refactor MemMap::MapAnonymous().

Remove the address argument from the shortcut overload and
introduce one more shortcut overload. This makes it easier
to find all uses where we pass non-null address hint.

Remove `requested_begin` parameter from some constructors
where we were always passing null. Rewrite some tests to
use the reservation API.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 118408378
Change-Id: Ibbbb96667e7cc11cf7fea119892463d8dbc9a8b5