Support sentinel values for CHAR LIMIT.

For common string placements (such notification titles), we should
have consistent CHAR LIMIT values that are applied universally.

Strings can opt-into using these consistent values with new sentinel
values which the "transconsole" tool will turn into real numbers
based on the real-world UI measurements in the linked bug.  The
currently measured values are:

NOTIF_TITLE=48
NOTIF_BODY=384
NOTIF_ACTION=24

DIALOG_TITLE=56
DIALOG_BODY=NONE
DIALOG_ACTION=64

TOAST=384

Bug: 110095706
Test: manual
Change-Id: I6bae3a1b5bb6a03cb50a25325b7eb75a535ee6c5