Display most recent activity in room list (#220)

* Create `RoomLastMessageFormatter` to produce readable room summaries.

* Add unit tests using Robolectric, fix bugs

* Add changelog

* Move `RoomLastMessageFormatter` back to `impl` module, allow it to receive an `EventTimelineItem` instead of `MessageContent`.

libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/LastMessageTimestampFormatter.kt

@@ -16,6 +16,6 @@
 
 package io.element.android.libraries.dateformatter.api
 
-interface LastMessageFormatter {
+interface LastMessageTimestampFormatter {
     fun format(timestamp: Long?): String
 }