Use LoadHook to create modules in java_sdk_library

Creating new modules in a mutator is dangerous, as other mutators
that need to see the new modules may already have run, in this case
the prebuilts mutator.  Move SdkLibraryMutator to a LoadHook instead.
Also moves registering the LoadHook mutator to testing.go so it is
registered for all tests.

Test: m checkbuild
Change-Id: I08bd76a0e6205d2ca27861058067a1562c339eed