Normalize output from aapt d xmltree

When using aapt dump xmltree to dump an XML which has a content element with newlines, the output contains a
newline. This makes it very difficult, sometimes impossible, to understand what is part of the content, and
what is the meta-data.

We now pass XML content through the same normalizer used for other tags.

Change-Id: I327321520fac563eb32aecaf796f2473866697fc