android: make some activity attributes private

They are only meant to be used internally. This reduces the risk of SDK users
sending garbage when we expect a specific type.

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetActivity.java

 
     protected static final String TAG = JitsiMeetActivity.class.getSimpleName();
 
-    public static final String ACTION_JITSI_MEET_CONFERENCE = "org.jitsi.meet.CONFERENCE";
-    public static final String JITSI_MEET_CONFERENCE_OPTIONS = "JitsiMeetConferenceOptions";
+    private static final String ACTION_JITSI_MEET_CONFERENCE = "org.jitsi.meet.CONFERENCE";
+    private static final String JITSI_MEET_CONFERENCE_OPTIONS = "JitsiMeetConferenceOptions";
 
     // Helpers for starting the activity
     //