[Mfill Android] Rename Action in accessory to BarItem

This CL shouldn't change any behavior or appearance at all.

The name action is generic and slightly misleading as it gives the
impression that every item in the accessory bar maps to a native,
recorded action.
This is not entirely true as the items in the bar can actually differ
from these which becomes more clear in https://crrev.com/c/1407194 when
bar items like the tab switchers make their debut.

Minor non-functional linter errors are fixed while touching these files.

Bug: None
Change-Id: I3f4484df440c5b8c7d399ebdd133c5c67bb071ab
Reviewed-on: https://chromium-review.googlesource.com/c/1406978
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622420}