resourceloader: Change cache keys to use first segment for grouping by purpose

resourceloader: Change cache keys to use first segment for grouping by purpose

Remove use of a complete "namespace" for grouping and instead use dashes
within the first key segment for that.

This way, we can leverage the various features WANObjectCache provides
for statistics relating to a particular kind of thing being cached.

These statistics are currently combined for all of resourceloader,
which isn't useful. Mildly related to T223647.

Change-Id: I5df69e46ac436d04e765726a36fc3eb70697a7ed