Use the "screen security" preference for the conversation list

It was previously only used for ConversationListActivity. This should be all
the locations (according to a grep for FLAG_SECURE).

FREEBIE
Only receive text/plain.

At the moment only text/plain is handled in RoutingActivity, so set filter in manifest accordingly.
This prevents #316. But at some point sharing text/calendar or better */* would be nice...

// FREEBIE
Merge pull request #1020 from lxgr/fix-screensec

Use the "screen security" preference for the conversation list
Merge pull request #1025 from DorianScholz/sharetextplain

Only receive text/plain.