Deal correctly with suggested print attributes

An app might suggest certain attribute for an print job. This patch
fixes

- PrintActivity should not silently overwrite the attributes with
  the default values
- Handle duplex mode in PrintAttributes similar to the other attributes.
  I.e. have an "unset" value (0). This is an API change. But the only
  use case for PrintAttributes was to pass them to print() and thereby
  the PrintActivity. This was broken (see above), hence we are changing
  and API that could have never been used.

Bug: 23629618
Change-Id: I43c25704497c799352fb806126dc93f6db4879f5