1) There is no longer a concept of "verified" or "unverified."
Only "what we saw last time" and "different from last time."
2) Let's eliminate "verify session," since we're all about
identity keys now.
3) Mark manually processed key exchanges as processed.
1) Consolidate all of the KeyCachingService interaction into a single
mixin. Activities extend delegates which call through to the mixin.
2) Switch Activity increment/decrement triggers from onStop to onPause
in order to account for some screen locks that don't stop activities.
1) Change all instances which use concatenation to build strings
with variables in them to use string formatting instead.
2) Extract all string literals from layouts and menus into strings.xml
3) Extract all string literals from code into strings.xml
