bmpblk: Add option to reference messages

Use '.format()' on every message to replace references to other strings
in the message. This way the same repeating string (like URL) can be
used in multiple messages. Both, generic and localized messages, can be
inserted.

Use placeholder and python format syntax for the replacements:
<ph name="placeholder">{msg[referenced_name]}<ex>info for
translators</ex></ph>

Fixed update_xtb.py so it can handle translation entries with embedded
XML nodes (placeholders are XML nodes).

BUG=b:488289370
BRANCH=main
TEST=Verified that all screens are displayed correctly

Change-Id: I8c4802ef2691754bc5b35259c019cb612a8c2a48
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bmpblk/+/7749457
Reviewed-by: Konrad Adamczyk <konrada@google.com>
Reviewed-by: Subrata Banik <subratabanik@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Subrata Banik <subratabanik@chromium.org>
Auto-Submit: Tomasz Michalec <tmichalec@google.com>
Tested-by: Tomasz Michalec <tmichalec@google.com>
GitOrigin-RevId: f0d21705f971c56e9bc72c9756732a8e12e8242f