Fix lingering UI bugs

[stevenzeck]

This PR fixes a few lingering UI bugs that have plagued the testapp for a bit now.

1. The Outline fragment (ToC, bookmarks, etc) now has a back button in the toolbar that returns to the reader screen without changing the page
2. The height of the toolbar is no longer being used to pad the UI. For screens that need that padding because the contents are underneath the toolbar, android:layout_marginTop="?attr/actionBarSize" should be added to the root element
3. When the insets were being applied, it was using statusBars instead of systemBars and caused the TTS screen to not render the control buttons properly. Think I caused that one.
4. The control buttons on the TTS screen resized to 48dp x 48dp
5. I also added a divider item decoration to the table of contents view, I think it makes it a bit easier to read through them. I can revert if it's not wanted.

[mickael-menu]

Looking good, thanks!