Better UX handling on identity key mismatches.

1) Migrate from GSON to Jackson everywhere.

2) Add support for storing identity key conflicts on message rows.

3) Add limited support for surfacing identity key conflicts in UI.

res/drawable/error_round.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:state_pressed="true">
+        <shape>
+            <corners android:radius="2dp" />
+            <solid android:color="#FFD32F2F" />
+        </shape>
+    </item>
+
+    <item>
+        <shape>
+            <corners android:radius="2dp" />
+            <solid android:color="#FFF44336" />
+        </shape>
+    </item>
+</selector>