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_indicator.xml

@@ -20,7 +20,7 @@
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
     <item android:gravity="start">
         <shape android:shape="rectangle">
-            <solid android:color="@color/colorAccent" />
+            <solid android:color="?colorAccent" />
             <padding
                 android:left="0dp"
                 android:right="@dimen/padding_xlarge" />