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_sheet.xml

@@ -21,6 +21,7 @@
     <item>
         <shape android:shape="rectangle">
             <corners android:radius="16dp" />
+            <solid android:color="?android:colorBackground" />
         </shape>
     </item>
 </layer-list>