Do not clear category tile cache in every reload.

Bug: 31781480
Test: manual

The cache is useful when setting summary text on tiles. If tile cache is
cleared and rebuilt, summaryloader won't find existing tile to update
summary.

The cache should only be cleared when entire resource config changes,
tracked by InterestingConfigChnages.

Change-Id: I3afcaba30b835f59ffaad9c27564f0345af00d66