Added blackout for recent apps screen setting
Blackout app in recent apps view using FLAG_SECURE

This has an unintended (possibly unwanted) side effect of preventing
screenshots. onCreateThumbnail is not yet called by android system.

Will have to add messaging as such to the preference.
Add warning dialog to Recent App blackout
Merge branch 'master' into feature/recentBlackout

Conflicts:
	TMessagesProj/src/main/java/org/telegram/ui/LaunchActivity.java
updating readme