Fixed Extra Checkable background on press for API <21

core/src/main/res/layout-ldrtl/md_listitem_multichoice.xml

@@ -29,6 +29,7 @@
         android:layout_width="wrap_content"
         android:layout_height="match_parent"
         android:button="@drawable/abc_btn_check_material"
+        android:background="@null"
         android:clickable="false"
         android:focusable="false"
         android:focusableInTouchMode="false"

core/src/main/res/layout-ldrtl/md_listitem_singlechoice.xml

@@ -29,6 +29,7 @@
         android:layout_width="wrap_content"
         android:layout_height="match_parent"
         android:button="@drawable/abc_btn_radio_material"
+        android:background="@null"
         android:clickable="false"
         android:focusable="false"
         android:focusableInTouchMode="false"

core/src/main/res/layout/md_listitem_multichoice.xml

@@ -17,6 +17,7 @@
         android:layout_height="match_parent"
         android:clickable="false"
         android:button="@drawable/abc_btn_check_material"
+        android:background="@null"
         android:focusable="false"
         android:focusableInTouchMode="false"
         android:gravity="center_vertical" />

core/src/main/res/layout/md_listitem_singlechoice.xml

@@ -16,6 +16,7 @@
         android:layout_width="wrap_content"
         android:layout_height="match_parent"
         android:button="@drawable/abc_btn_radio_material"
+        android:background="@null"
         android:clickable="false"
         android:focusable="false"
         android:focusableInTouchMode="false"