Don't fail if the target module is disabled in dex2oat tool

dependencies.

dexpreopt.RegisterToolDeps runs late after prebuilt dependencies have
been resolved, and there's special code in dex2oatPathFromDep to
resolve the prebuilt from the source module. However, if the source
module is disabled then the dependencies check in validateAndroidModule
will complain, so we need to disable that check in this particular
situation.

Also add a comment to explain why dexpreopt.RegisterToolDeps needs to
run so late.

Test: m nothing
Bug: 145934348
Bug: 172480615
Change-Id: Ibc673303d0336768fa23261a2068e91a08f46a30