If we don’t executePendingTransactions we might still access the overview fragment
while a replacement operation is in the works. This will lead to two
conversationfragments opening.

This reverts commit e528b9f5df59f7b49ae18c73396bd56525493e28.

I was originally convinced by the argumentation (quote always in same place)
but testing this out for a while really seems to break 'last correct' for me.

I use that way more frequently that quote

- Add setting to prevent screenshots
- Enforce using FLAG_SECURE in onResume for each activity

This is a proof of concept.  It catches tel: from browser, but also taps on call
button in contacts, etc, and rewrites to a Cheogram JID.  It does not query the
user's roster for gateway options, or ask the gateways to convert the URI, but
simply assumes Cheogram format and jams it in.  It also does not initiate a
call, but simply uses the default XmppUri action which will be to either add to
roster or open conversation.

* 'jid-in-calls' of https://github.com/benharri-forks/Conversations:
  display jid in calls

Import some build system patches from fdroid.

since 0 is a valid file size we should use null to indicate absence