Fix BackupPrefsFragmentTest path

[EmmanuelMess]

it seems that the test hardcoded path might not exist in the emulator.

[EmmanuelMess]

@VishnuSanal I'm thinking that the Espresso test should only test the interface part of the operation, not the actual preference loading, that would prevent needing to get the actual home path from fragment from the activity from the intent. And then we can test the actual behaviour from Robolectric. This also reduces the amount of emulator time somewhat.