fix #1935: move EDITOR_CREATED_POST tracking and add a source property

fix #1935 

There are actually 5 ways to create a post from the Android app:
1. from the Post List
1. from the Media Library
1. from QuickPress
1. from QuickPhoto or QuickVideo
1. from an external app (using the "Share with..." Android feature)

cc @sendhil 
Perhaps this is nitpicky, but it seems odd not to use `else if` in these if statements.