Add more "extended opcode" structure to libdex.

Although we don't yet generate any of the extended opcodes, this
change makes it a bit easier to add them.

In particular, we now differentiate between the raw opcode in a code
unit and an associated "packed opcode number." The packed opcode space
is densely populated in the range 0x000-0x1ff (though there will still
be a few unused slots), whereas the raw opcode values are sparse
throughout the range 0x0000-0xffff.

The OpCode enum is redefined/clarified to have packed, not sparse,
opcode values.

Change-Id: Ie3208a258648fbf044d344646f66c49ad24c31b2