🔥 Drop support for API 19 (KitKat)

I'm fed up with all the extra effort for supporting an outdated
Android version. Apologies for the few users that are now excluded,
but I feel like it's just not worth the hassle.

app/src/main/res/xml/preferences.xml

@@ -34,7 +34,6 @@
         <SwitchPreferenceCompat
             android:defaultValue="false"
             android:key="require_charging"
-            android:enabled="@bool/is_post_api_21"
             android:summary="@string/require_charging_summary"
             android:title="@string/require_charging" />
         <SwitchPreferenceCompat