Defer \n quoting generatedContents until creating the ninja rule

Previously, when writing to generatedContents \n characters were
quoted (replaced with \\n) so as to allow them to be preserved through
ninja/rsp/bash and were unquoted (replaced \\n with \n) just before
redirecting to the output file. That meant that any code which wanted
to access the contents for other purposes, e.g. testing had to unquote
\\n.

This change moves the quoting to be part of the code that generates the
ninja rule which simplifies any other code that has to access the
contents.

Without quoting the generated Android.bp files are not formatted
properly, are all on one line and completely unreadable.

Bug: 156286550
Test: m art-module-sdk and check generated Android.bp file to make sure
      it is properly formatted.
Merged-In: I768c3b96ed08a3daf251730e2a10d9d72338c49a
Change-Id: I768c3b96ed08a3daf251730e2a10d9d72338c49a
(cherry picked from commit 1110827b)