adb: Fix PTY logic for non-interactive shells.

Change `adb shell` so that interactive sessions use a PTY but
non-interactive do not. This matches `ssh` functionality better
and also enables future work to split stdout/stderr for
non-interactive sessions.

A test to verify this behavior is added to test_device.py with
supporting modifications in device.py.

Bug: http://b/21215503
Change-Id: Ib4ba40df85f82ddef4e0dd557952271c859d1c7b