clear cache

This PR was implemented to clear caches under one specific Account directory (the currently login account).

Seadroid caches files on external SD card, caches of big size like photos or files are placed under specific Account directory. This PR assumes that user want to clear those big size files because user may only want to upload them to the server.

@lins05  account always be null pointer here, so the clear cache module can not be tested. I plan to test this after the Sign out #200 was merged because in this PR getCurrentAccount was refactored to AccountManager and used another SharedPreference