Add support for disabling display scaling for development.

Added two new options to the wm command.

1. Set the screen size based on dips rather than pixels using the
current screen density.

eg. adb shell wm size 320dpx320dp

2. Disable automatic scaling of the contents of the display.
When combined with the previous command, this is useful for seeing
how the UI would behave if the screen remained at its current density
but changed physical size.

eg. adb shell wm scaling off

Bug: 19899223
Change-Id: I545f893ba4861494e995cf0457ebeba1050d28dc