Show 'UPDATE' text for the save button when editing an uploaded post.

Button still works the same way as it did before for local drafts.

Fixes #1422
Renaming update_location string to update_verb to make more contextual sense.
Make one db post save call instead of two when saving the post.
Added a null check for the post object in savePost(), just in case.
Set save button to 'Schedule' if the PostStatus is scheduled.

Renamed the save methods in the fragments to 'update' since they are only updating the post object now.
Missed on condition: Button should read 'Update' for all post statuses in an uploaded post, except if it is scheduled.

Also added updatePostSettingsAndSaveButton() method to fix some DRY.
Merge pull request #1425 from wordpress-mobile/issue/1422-upload-button-text

Show 'UPDATE' text for the save button when editing an uploaded post.