When inserting an cached resource, or removing a region, remove least-recently used resources and tiles, not used by offline regions, until the used database size, as calculated by multiplying the number of in-use pages by the page size, is less than the maximum cache size minus 5 times the page size.
In addition, OfflineDatabase may be configured to ignore cache puts of individual resources larger than a certain size.
This policy is similar but not identical to the former SQLiteCache policy:
* It accounts for offline, by exempting resources required by offline regions from eviction.
* It must delete from two tables (resources and tiles), rather than one. Currently the strategy is naive: evict 50 rows at a time from each table.
* It makes maximumCacheSize and maximumCacheEntrySize completely independent. The SQLiteCache implementation evicted when `usedSize > maximumCacheSize - 2 * maximumCacheEntrySize`. This evicts when `usedSize > maximumCacheSize - 5 * pageSize`.
* It uses a non-unlimited default value for maximumCacheSize: 50 MB. We should have always had a limit in place; "a cache without an eviction policy is a resource leak".
I regenerated assets.zip so that all file paths have an `assets/` prefix, as the Android AssetFileSource implementation asserts, and removed `TEST_DATA` from the paths.
* Move asset:// URL handling to DefaultFileSource.
* AssetFileSource implements FileSource interface and follows familiar implementation patterns.
* Move default implementation to platform/default, zip implementation to platform/android.
* Don't bother with modified / expires / etag -- assets are not cached so it doesn't matter.
* Don't bother with interleaving individual IO calls on the implementation thread. That adds a lot of complexity for very little benefit.
