Integrate QR Code library instead of requiring the app to be installed, also due to security reasons

OpenKeychain/src/main/res/values/strings.xml

@@ -395,6 +395,7 @@
     <string name="import_qr_code_wrong">"QR Code malformed! Please try again!"</string>
     <string name="import_qr_code_too_short_fingerprint">"Fingerprint is too short (&lt; 16 characters)"</string>
     <string name="import_qr_code_button">"Scan QR Code"</string>
+    <string name="import_qr_code_text">"Place your camera over the QR Code!"</string>
 
     <!-- Generic result toast -->
     <string name="view_log">"View Log"</string>