Avoid auto-scroll after deleting a message

See WhisperSystems/TextSecure#139

This fix involves setting transcriptMode to normal on conversation view,
which resolves the issue but comes with the side effect that if you
have scrolled away from the bottom of the list, new incoming messages
will not trigger auto-scroll as they arrive.

res/layout/conversation_fragment.xml

@@ -11,7 +11,7 @@
               android:layout_height="0dp"
               android:layout_weight="1.0"
               android:drawSelectorOnTop="true"
-              android:transcriptMode="alwaysScroll"
+              android:transcriptMode="normal"
               android:scrollbarAlwaysDrawVerticalTrack="false"
               android:scrollbarStyle="insideOverlay"
               android:stackFromBottom="true"