navigation: fix greying out the app icon if not enabled

Commit 0aead42f ("navigation: Add is available (#1847)") added the
ability to draw the app icon in grey and in a disabled state when some
prerequisits were not met. Only the Navigation app was using this mechanism
due to its icons being stored in the external memory and possibly missing.

Commit 63e0c4f4 ("Application selection at build time") broke this by
always setting the state as true:
      for (const auto& userApp : userApps) {
        apps[i++] = Screens::Tile::Applications {userApp.icon, userApp.app, true};
      }

Fix this by creating an isAvailable() strcuture in the app classes, similar
to how the Watchfaces handle the same problem of checking availability.