implementation for setting the passphrase by @abika

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>

Conflicts:
	app/src/main/java/org/kontalk/authenticator/Authenticator.java
	app/src/main/java/org/kontalk/ui/PreferencesActivity.java
	app/src/main/res/values/strings.xml
Change key passphrase preference, thanks to @abika (UNTESTED)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>