Only unwrap { generated: wrapper } based on context.

Wrapper paragraphs should only be unwrapped if they are the first
element in their parent - or if there is a block level element separating
them from the previous unwrapped paragraph.

Empty paragraphs should only be unwrapped if they are empty and the
last element in their parent.

Also in this commit is a simple test for IndentationAction.decrease().

Bug: 45590
Change-Id: I1f47d12db6d57d984fd4607f667a3b62c53f3dd6
Merge "Only unwrap { generated: wrapper } based on context."