Add java_boot_libs to sdk

The build has some implicit dependencies (via the boot jars
configuration) on a number of modules, e.g. core-oj, apache-xml, that
are part of the java boot class path and which are provided by mainline
modules (e.g. art, conscrypt, runtime-i18n) but which are not otherwise
used outside those mainline modules.

As they are not needed outside the mainline modules adding them to
the sdk/module-exports as either java_libs, or java_header_libs would
end up exporting more information than was strictly necessary. This
change adds the java_boot_libs property to allow those modules to be
exported as part of the sdk/module_exports without exposing any
unnecessary information.

Some points to note:
* The java_import has to have a valid file for the src property
  otherwise it will be disabled.
* The src property is supposed to reference a jar file but the
  java_boot_libs property will make it reference an empty file (not
  an empty jar) so that any attempt to use that file as a jar, e.g.
  compiling against it, will cause a build failure.
* The name of the file passed to the src property should make it
  clear that the file is not intended to be used.
* The test makes sure that only the jar file is copied to the
  snapshot.

Test: m nothing
Bug: 171061220
Change-Id: I175331e4c8e3874ab70a67cdc2f76ed1576e41eb