Fix intermittent slowness in resolver activity towards end of day.

The resolver calls UsageStatsManager.queryUsageStats() to get package
usage information. This causes all XMLs of the selected bucket to be
read, which can be of significant size. However, the majority of that
size is due to usage event information, which queryUsageStats() isn't
interested in, so it'll parse a lot of data just to throw it away later
on.
Solution: Don't parse the portions of the XMLs we aren't interested in.

Change-Id: Ia2595201c9fcceab2778618fd0cccfb19ad4ed8d