ART: Set default visibility to protected

Set default visibility of symbols to protected. This allows the
linker to optimize internal calls and helps avoid plt calls.

Make almost all assembly stubs hidden, as -fvisibility does not
seem to apply to them. Change the assembly tests accordingly. Also
allows to clean up previous hacks to avoid plt calls.

Bug: 16974467
Change-Id: I9030dcf6116251f434f94a2b08e56e12085af652