Unhide Resources APIs for getting resources at specific densities.

This adds Resources.getDrawableForDensity() and
Resources.getValueForDensity().  These are needed for applications
to correctly retrieve larger icons such as in launcher when
running on a tablet.  We had already exposed the APIs to tell the
application which density to use for app icons on the current
device, but didn't unhide these APIs that allowed you to
actually retrieve them.

This is safe to do without introducing a new API level (as long
as we do it soon) because we know these APIs already exist in
Android 4.0, and there is no reason for anyone to be removing
them when building a device.

Change-Id: I5138e5dc908197b66a98d20af73c5374cb5d41d3