android: Force the two line button to be focusable

On newer Android versions (8+) this does not seem to be necessary (adding
the onClick handler also sets "clickable" and that in turn seems to make
it focusable), however, for older releases it is (tested with 7.1.1
keyboard navigation just skips over the button).  This was seen on a
Fire TV stick.

src/frontends/android/app/src/main/res/layout/two_line_button.xml

@@ -19,6 +19,7 @@
     android:minHeight="?android:attr/listPreferredItemHeight"
     android:background="?android:attr/selectableItemBackground"
     android:mode="twoLine"
+    android:focusable="true"
     android:padding="10dp" >
 
     <TextView