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/layout/sheet_base.xml

@@ -20,7 +20,7 @@
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
-    android:background="?android:colorBackground"
+    android:background="@drawable/bg_sheet"
     android:orientation="vertical">
 
     <FrameLayout