Global resources counter and occupied disk space info

This feature is conceptually simple, but requires some smart structure in fact.
Architecturally, we should modify the counter every time we index a resource.

Visually, these counters should be placed on the _folders screen_.

Start by locating the resource-indexing entry point and the _folders screen in the Android app. Trace where resources are indexed and determine how that counter can stay updated, then verify that the folders screen displays both the global resource count and occupied disk space accurately.

Written by the indexing model from the issue text.