Links in Echo notifications should be internal

The i18n team has previously requested that <span class="plainlinks">
be moved outside the i18n files, and be applied somewhere in the render
pipeline.  We can and will continue to do that for all instances where
we control that pipeline, unfortunatly the message building for Echo
notifications is generic to the point that it is not possible to externally
wrap them.

This patch re-adds <span class="plainlinks"> only to i18n messages used
by the Echo notifications engine.

Bug: 60867
Change-Id: Ib2221cf955e5a9be4b94d6f818803d6bbe743de0
Merge "Links in Echo notifications should be internal"