Fix onboarding_warning crash

This crash references #132 (closed) and it happens when action_onboardingBatteryFragment_to_onboardingWarningFragment is invoked in BrowserActivity.kt. Since BrowserActivity.kt serves as the parent/host activity for SettingsFragment.kt, clicking the "Disable battery optimization" setting item triggers a crash. This happens because, in this instance, the source fragment is not OnboardingBatteryFragment.

To resolve this, I've introduced a global action decoupled from a specific source fragment. This will prevent crashes when triggering the action from any context.