ui: show human-friendly last interaction date and sumary

...pp/src/main/java/cx/ring/client/ConversationActivity.java

...app/src/main/java/cx/ring/fragments/CallListFragment.java

...ndroid/app/src/main/java/cx/ring/history/HistoryCall.java

...droid/app/src/main/java/cx/ring/history/HistoryEntry.java

...android/app/src/main/java/cx/ring/model/Conversation.java

ring-android/app/src/main/res/layout/item_calllist.xml

ring-android/app/src/main/res/layout/item_textmsg.xml

ring-android/app/src/main/res/values/strings.xml