Alter device.freeDisk to collect usable space in data directory

[fractalwrench]

Goal

device.freeDisk currently collects the size of the external storage directory, which is not helpful
when debugging errors that occurred in a low disk space situation. This changeset gets the /data directory and calculates the usable space in bytes (to non-root users) which is a more useful measure of the device's disk space.

Tests

Verified that the reported value was approximately 5.1Gb on a physical Nexus 4 device.

[kattrali]

I think I understand, but I don't know what the tradeoffs are here.

• Is anything being lost here by changing implementations?
• Was the new implementation not available as an option when calculateFreeDisk() was initially implemented, or did it serve some other purpose?
• Should the old value (or the new value) be present in metadata as a separate key?
• The old implementation appears to display the smaller of two values, comparing internal vs external available space. Is this no longer feasible or not important?

[fractalwrench]

The old calculation for freeDisk calculated the total amount of disk space and is therefore misleading in the way we present that information, as the name suggests its the amount of available free bytes, rather than the total.

Android distinguishes between internal storage (which is always present) and external storage (which can be unmounted/detached from the device). The majority of apps write their files to internal storage, because writing files to external storage requires additional permissions that must be added to the manifest. Additionally, there are upcoming changes in the form of scoped storage which will make use of external storage less desirable for most use-cases.

This change intends to capture the most useful information by default. Although I agree it's worth thinking about whether it makes sense to collect other information around external storage disk space by default too as some separate metadata tab.

review points addressed