adbd: spawn login shell when run without a command.

Match openssh's behavior with regards to login shells.

Bug: http://b/76116378
Test: `adb shell -T set -o` vs `ssh localhost -T set -o`
Test: `adb shell -tt set -o` vs `ssh localhost -tt set -o`
Test: `echo set -o | adb shell -T` vs `echo set -o | ssh localhost -T`
Test: `echo set -o | adb shell -tt` vs `echo set -o | ssh localhost -tt`
Change-Id: I6ed69b43e2ac79c947c242948b5617ece08a713e