adb: non-interactive shell stdin.

Non-interactive `adb shell` previously only read from the remote shell,
but we want it to write as well so interactive and non-interactive
shells can both send data. With this CL, we can now do:
  $ echo foo | adb shell cat
  foo

This is primarily usable with newer devices that support the shell_v2
features. Older devices will receive stdin but the shell will still
hang after all input has been sent, requiring user Ctrl+C. This seems
better than closing communication altogether which could potentially
miss an unpredictable amount of return data by closing too early.

Known issue: non-interactive stdin to a PTY shell isn't reliable.
However I don't think this is a common case as ssh doesn't seem to
handle it properly either. Examples:
  * echo 'echo foo' | adb shell
  * echo 'foo' | adb shell -t cat

Bug: http://b/24565284
Change-Id: I5b017fd12d8478765bb6e8400ea76d535c24ce42