Implement App Hiding

This commit allows F-Droid to hide itself from the laucher. It can be hidden either as response to a panic trigger or as a manual action by long pressing the floating search button. The latter needs to be explicitly enabled in the settings.

Once hidden, a semi-functional fake calculator app appears in the launcher that can be used to bring F-Droid back by entering a pre-defined PIN.

This MR does not add any new fragments :P