adb: fix subprocess termination for legacy shell.

http://r.android.com/166419 changed `adb shell` behavior to not
allocate a remote PTY for non-interactive commands, but adbd relied on
having a PTY to properly terminate the subprocess.

One impact of this is that when using older versions of adb or passing
the -x flag, `adb screenrecord` wasn't properly terminating and closing
out the video file.

This CL restores the old behavior for legacy shell connections: always
use a PTY, but put it in raw mode if the client is doing local PTY
input/output processing itself.

Bug: http://b/26742824
Change-Id: I9ee630c0ff0d2d6a0db367387af7123deea79676