Show a warning when the battery optimisation is active for the app

[GrazianoCapelli]

Some systems / brands (starting from Marshmallow) adopt an aggressive behaviour with the background apps, specially when the battery optimisation is active.
We should show a warning to inform the users that the app is battery optimised.
The warning doesn't block in any way the app and, as soon as the app obtains the FIX, the warning disappears.

On some other systems / brands the app works well also when battery optimised; maybe we should add the possibility to dismiss the warning (if dismissed, should it never appear again?)...

TO DO LIST:

• v3.1.4 - Add the Warning to the GPS Fix Tab (as a first implementation we'll use the @string/dlg_app_killed_description)
• v3.1.4 - Add a small X button to allow the user to dismiss the warning (only for the session)
• v3.2.0 - Add a new string to better describe the battery optimisation warning

[GrazianoCapelli]

On some other systems / brands the app works well also when battery optimised; maybe we should add the possibility to dismiss the warning (if dismissed, should it never appear again?)...

I chosen the conservative approach for now: the warning is dismissed only for the current session; when the app is closed and reopened, the warning appears again. I chosen this approach because:

1. Even if the app works well with optimisation active, its deactivation does not increase the battery usage. The battery optimisation should be disabled also in these cases;
2. The warning doesn't block anything, and disappeas when the FIX is found;