on initialisation calculateFreeDisk was indirectly invoked, which performs IO on the main thread.
This change avoids calls to getDeviceData() which would trigger these calculations, and adds getters
for the necessary fields instead. This stops StrictMode violations being reported when initialising
the SDK.

JUnit offers a RunListener, which has a method that is invoked after each test is completed.
Specifying this is preferable to using @after for teardown, as we can cancel asynchronous tasks from
one place only in this manner.

By default, we should cache the result of whether the device is rooted or not, as the result is
unlikely to change between app launches and the calculation requires IO.