Header population cleanup

* Show the app name for minimized notifications without titles.
* Don't show the summary separator when the summary text is an empty string.
* Use TextUtils.isEmpty for clarity & add comment.
* Make visibility reset of app_name_text consistent with others.

Bug: 163626038
Test: manual
Change-Id: I682a8054abb80f24044a2a9c204dfadd5e940860