Enable F841 and fix all issues.
Flake F841 is:
F841 local variable 'XXX' is assigned to but never used
Also fix in changed files some occurences of:
E128 continuation line under-indented for visual indent
H404 multi line docstring should start without a leading new line
H405 multi line docstring summary not separated with an empty line
Change-Id: Ifb08a82d05ac15c8e90f214a96e73d82ff850018
Partial-Bug: #1333290