add optional strict offline checking for calls

src/main/java/eu/siacs/conversations/Config.java

src/main/java/eu/siacs/conversations/entities/Presences.java

...java/eu/siacs/conversations/services/CallIntegration.java

...versations/services/CallIntegrationConnectionService.java

...in/java/eu/siacs/conversations/ui/RtpSessionActivity.java

...a/eu/siacs/conversations/xmpp/jingle/RtpEndUserState.java

src/main/res/values/strings.xml