Enable hacking H301 and disable H304, H302

Enable hacking H301: one import per line. H304 and H302 check enables too,
so we temporarily disable it since it does not pass.

Fix imports in files below.

Add # noqa to migration scripts.

Change-Id: I6a8d4dcd9c3195d2848f218aafe304b1240ab60c
Merge "Enable hacking H301 and disable H304, H302"