Add a way to persist filled-in data

[EwoudVanPareren]

Right now, when the Android application (and Activity) is closed by the system in the background, the contents of the filled-in data (specifically, the data in MainScreenModelForm) is lost.

Compared to the convenience of the Desktop version's ability to stay alive in the tray, this is a little inconvenient. It would be nice to be able to match that ability in some way.

One downside to this is that it would likely mean persisting the Master Password to the device storage. While this is not a big issue in practice (the system should not leak the data), it does violate the principle of never committing the Master Password to storage (or entrusting it to an off-app system). For this reason, it should not be the default behavior. It should be manually and deliberately enabled by the user.

Another downside is that it could allow anyone who gains access to the device to easily access the app and make unauthorized use of the Master Password. For this reason, the option to persist the filled-in data should only be available when #6 is enabled as well.

And finally, a choice should be made for the scope of the persistence.
Should the filled-in contents be persisted unless the Activity is specifically closed (through the back button or by closing it on the recent activities screen)? Or should it persist indefinitely, regardless of how the Activity is opened and closed?
The former is likely to cause some confusion for users who are not familiar with the distinction between closing an Activity and leaving it in the background. The latter is less secure.