Differentiate between no dist tag and an empty dist tag

Change https://r.android.com/1335521 added tag property to the Dist
struct so that it could be used to select one of a number of different
output files to copy to the dist instead of the single file that the
module type made available for dist. The output files were selected
by passing the tag to OutputFiles(tag).

Module types that wanted to support this new approach had to explicitly
set AndroidMkEntries.DistFiles = GenerateTaggedDistFiles(module).
Unfortunately, doing that had a side effect of changing the behavior of
dist entries without a tag.

That was because the change treated a tag that was not specified, as
being the same as "". So, prior to the change no tag meant use the
default dist file but after it meant use the paths returned by
OutputFiles(""). That changed the behavior of the java.Library type
which affected the behavior of the android_app module type.

Prior to the change the java_library would make the
Library.outputFile available for dist when no tag was specified. After
that change it would make Library.outputFile plus
Library.extraOutputFiles. The latter is usually empty except for
android_app which adds some extra files into there which will now be
copied to the dist. That change may have been intentional but there
was no mention of it in the change or the bug. Even if it wasn't
intentional it may still be beneficial.

Any module type that wants to add support for tags in dist runs the
risk of introducing similar changes in behavior. This change
differentiates between the tag not being set and the tag being set to
"" to avoid that possibility and to make the default behavior
explicit for those module types that have switched.

It does so as follows:
* Adds a DefaultDistTag constant that is used when the tag is not set.
  It is a string that is unlikely to be used as an actual tag as it
  does not start with a . and uses some special characters.
* The DefaultDistTag is used in MakeDefaultDistFiles(paths) to indicate
  that the supplied paths are the default ones and and also in
  GenerateTaggedDistFiles() for Dist structures that have no tag
  property set.
* The DefaultDistTag is passed to OutputFiles(tag) just in case the
  module type has explicitly defined the paths to associate with that
  tag in there. If it has then it overrides the legacy behavior. If it
  has not then it is just ignored and falls back to using the previous
  behavior.
* The java.Library.OutputFiles(tag) method explicitly handles the
  DefaultDistTag and returns Library.outputFile for it which restores
  the behavior from before the change that added dist.tag support.
* Similar change was made to apexBundle.OutputFiles(tag) in order to
  preserve its previous behaviour.
* The customModule used by TestGetDistContributions has been modified
  to also preserve its previous behavior after this change.

Test: m nothing
      m dist sdk - before and after this change, compare result to
      make sure that there are no significant differences.
      Test the effect on the apex by following instructions in
      http://b/172951145
Bug: 174226317
Change-Id: Ib8f0d9307751cc2ed34e3d9a5538d3c144666f6d