Fiddle with default densities to try to sanitize the API.

An issue with the density API is that bitmaps assumed the old default density,
so new programs would have to explicitly set the correct density for every bitmap
they create.

This is an attempt to fix that situation, by define the default density of bitmaps
to be the main screen's density, except for old apps where it is the original default
density.

Actually implementing this is not so great, though, because the Bitmap constructors
can't really know anything about who is calling them to know which density to use.
So at this level the compatibility mode is defined per-process -- meaning the initial
package loaded into a process defines the default bitmap density, and everyone else
loaded in later on has to live with that.

In practice this shouldn't be much of a problem, there shouldn't be much mixing of
old vs. new apps in a process.  It does mean that, going forward, if a developer is
going to use shared user IDs for this, they will need to make sure either that all of
their apps are in the same compatibility mode, or that their code explicitly sets the
density of bitmaps it receives.  This isn't all that great, but I think it is worth
the benefit of allowing people who write modern apps to not have to deal with bitmap
densities.

This change also does some cleanup of the density management (making sure to always
copy over bitmap densities, etc) and adds java docs to explain the various ways
density is set and used by the system.

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