Drop support for kitkat; Bump minSdk to 21

This has been dead for a while already, remove the code and bump minSdk to a
version that actually works

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

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

@@ -24,7 +24,7 @@
             android:shape="ring"
             android:thickness="12dp"
             android:useLevel="false">
-            <solid android:color="@color/colorAccent" />
+            <solid android:color="?colorAccent" />
         </shape>
     </item>
 </layer-list>