ART: Add FdFile constructors

Make Open protected, and expose constructors instead. Add a move
constructor and move assignment operator.

Add OS functions that return the FdFile non-pointer version.

Add tests.

Bug: 21192156
Test: m test-art-host
Test: m test-art-target (shamu)
Change-Id: I83e390edde7cd37c900e9d5c3e4d21da22981b3f
6 files changed