Create prebuilt api modules in load hook not mutator

An attempt to reference one of the prebuilt modules directly from an
Android.bp file highlighted a bug. The prebuilt_apis module used a
mutator to create filegroup and java_import modules for a set of api
versions. That mutator ran after the "prebuilts" mutator which handled
the renaming of prebuilt modules when the matching source module was
not present. That meant that the only way to reference the module was
by explicitly adding the prefix "prebuilt_".

This change fixed that bug by creating the modules in a load hook rather
that a mutator. This ensures that the prebuilt modules are present well
before the "prebuilts" mutator is run. Removing the mutator also removes
an unnecessary traversal of the whole dependency tree.

It also updated the documentation to explain that it creates
java_import modules as well as the filegroup modules.

Bug: 153649372
Test: m droid
Change-Id: Id44dce8ca9968ae903345df6ef1c4f1be9cb76c5