Fix a number of lint issues (#2)

* Fix systemUiVisibility deprecation warnings

This commit extracts setSystemUiVisibility as
an extension and uses the new API depending on
the Build.VERSION

* Fix splash fragment vector issues

Clip-path isn't available before V24 and
width/height should not exceed 200 dp.

* Remove unused disclaimer fragment header

* Remove unused ic_launcher_round

* Ignore IconMissingDensityFolder lint

Ignoring it because all the images are vector

* Ignore UnusedAttribute lint

It warns about networkSecurityConfig which
is used only in debug builds to allow
user SSL certificates. Lint says it's not
available before v24. But it doesn't matter
since before v24 the user SSL certificates
were allowed by default.

* Use plurals for disclaimer count down

app/src/main/res/drawable/ic_splash_screen_background.xml

@@ -1,11 +1,10 @@
 <vector xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:aapt="http://schemas.android.com/aapt"
-    android:width="360dp"
-    android:height="640dp"
+    android:width="113dp"
+    android:height="200dp"
     android:viewportWidth="360"
     android:viewportHeight="640">
   <group>
-    <clip-path android:pathData="M0,0h360v640h-360z" />
     <path android:pathData="M0,0h360v640h-360z">
       <aapt:attr name="android:fillColor">
         <gradient