Cache storage values for fast loading.

If the user moves away from the storage fragment and returns, this
allows us to use cached data from the previous calculation. If the data
is > 1 minute old, we consider it stale. Otherwise, we can bypass the
loading screen.

Fixes: 37923463
Test: Settings Robotest

Change-Id: I7650d4d742852f8d447878c077b9190bc0a0bb22