[android-test] tests for Back/Escape navigation #116

related opencloud-eu/android#226

File list Back navigation:

• In a subfolder with the FAB menu closed, pressing Back goes to the parent folder.
• At the root with the FAB menu open, pressing Back closes the FAB menu and stays on the file list.
• At the root with the FAB menu closed, pressing Back closes the app (the activity is finishing or destroyed).
• In a subfolder with the FAB menu open, pressing Back closes the FAB menu and stays in the current folder.