soong: Give priority to modules in exported namespaces for bootjars

* Looking for modules that provide a boot jar is currently
  done by module name match only.  This breaks when you
  have multiple copies of the same module in your build
  tree (eg in another device).

* Add a new mutator that marks the modules that should be used
  for bootjar content.

* The logic is simple: give priority to modules that are in
  exported soong namespace.

Change-Id: I2476174b892475c14a9f10b5e66b8496186f81c0