Add a window dump for uiautomator

There's no good way to dump all the windows and views that
uiautomator sees, so added one. Otherwise its a pain to
do multi-window debugging.

This could also possibly be used by various hierarchy
viewers that currently are only able to look at the active
window.

Bug: 151632128
Test: adb shell uiautomator dump --windows
Change-Id: I20b45507584e35ef5d725691b2ffb060bcfa18db