fix(scanner): use explicit padding attrs (paddingHorizontal/Vertical are API 26+, minSdk 21)

Author: hawkff

app/src/main/res/layout/layout_scanner.xml

@@ -26,8 +26,10 @@
         android:layout_gravity="center_horizontal|top"
         android:layout_marginTop="120dp"
         android:background="#66000000"
-        android:paddingHorizontal="16dp"
-        android:paddingVertical="8dp"
+        android:paddingLeft="16dp"
+        android:paddingTop="8dp"
+        android:paddingRight="16dp"
+        android:paddingBottom="8dp"
         android:text="@string/scan_qr_hint"
         android:textColor="@android:color/white"
         android:textAppearance="?attr/textAppearanceBody2" />