Fixed autosave crashes when saving to external files on some Android devices

Modding:
- Replaced semi-working policy branch restriction with countable + Unavailable
- Negative stat percentages from buildings displayed properly in city screen.

Experimental UI animations change. - By k-oa

By EmperorPinguin:
- Add pillage yield uniques 
- Add game progress modifier

By SomeTroglodyte:
- Minor refactor: Save/Load game loading image uses `LoadingImage` 
- Mod download robustness