DropBoxManagerService: Don't store redundant information

Filenames can be calculated from other fields, so there's no need to
store them.

This will reduce the average EntryFile size from 160b to 28b, so in theory
it could save 132 KB for 1000 entries.

- Also switched from HashMap to ArrayMap, which should help a bit too.

- Also fixed unit tests and added more.

Bug: 20890386
Test: bit FrameworksServicesTests:com.android.server.DropBoxTest
Test: Boot, check the dropbox directory, run dumpsys dropbox, reboot and repeat.

Change-Id: If567750f478318acd621864d1d4ef2ed41f214bd