Cache size

The current cache size is 10Mb which is way to small for any larger photo collection.
There is a PR (#144) which allows to manually resize the cache. I think this is not the best idea as the user has to do it manually, so this might be there as a fallback.

In https://github.com/nextcloud/android/issues/177#issuecomment-236945762 this was proposed:
- \>=40% of the internal storage available: set cache size to 5% of internal storage size of the device (cache is always in internal storage)
- < 40% of the internal storage available: set cache size to 10% of the remaining space available but at least 100MByte (which still gives you enough space for thumbnails and resized images)

Every time the apps starts the cache size will be checked and adjusted.