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/bg_bottomsheet.xml

@@ -23,7 +23,7 @@
             <corners
                 android:topLeftRadius="8dp"
                 android:topRightRadius="8dp" />
-            <solid android:color="@color/colorAccent" />
+            <solid android:color="?colorAccent" />
         </shape>
     </item>
 </layer-list>