Add opcode-emission backward compatibility.

Add opcode-emission backward compatibility.

This makes it so that when you pass dx --target-api=N, where N is an
API level representing Honeycomb or earlier, dx will not emit any of
the new extended opcodes.

N is currently baked into the code as 12 or larger being
post-Honeycomb, but it is subject to change if there are more revs of
the API under the Honeycomb umbrella (which wouldn't be surprising).

Bug: 4094709
Change-Id: Iaf5177f179b22586bcf806ecb53de20b6e989777