Add path to get different DPI drawables

Allow a caller to request a different density than their current display
allows. This can mean a device displaying mdpi can get a resource that's
in hdpi and have it pretend to be in mdpi resolution. If a drawable
that's returned is not in the requested density, it will set it at the
appropriate density to be scaled up later on.

The API for this is hidden currently.

Bug: 3134688
Change-Id: I6c3908cbdef4907b8d3f1576df9e3b0e7af1755a