Enables H404 check (multi line docstring) in flake8

This commit ensures docstring follows the docstring convention.

Also enables H301 (one import per line) in flake8.
There is already no violation the source codes.

Change-Id: Icf6ce5bae40b2e6492c060a28587d825a1837b43
Merge "Enables H404 check (multi line docstring) in flake8"