Pass $TERM to the device.

Unfortunately, this isn't backwards-compatible with the current shell
protocol because we made unknown shell: arguments errors. We could try
to commit the change to make them just warnings first, but how would
we know when everyone was running adbd with that change? Bumping the
protocol version doesn't help because that only affects the code running
on the host. And although we could add another feature to the reported
features, since shell_v2 is still in development, that doesn't seem
worthwhile.

Bug: http://b/25601436
Change-Id: I12b81aa656cd25b91d14ef691dcbd2b7dab49535