Remove const from key type in TemplateExpressions lib

The const qualifier is redundant as map keys are immutable by
definition. The only effective difference is that it is treated as a
different type from the non-const-qualified type, which can cause
unnecessary copy operations.

Bug: 384765582
Change-Id: I1d5ee759f9de16c3ad269c0ab70e2438dccfff07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6221795
Commit-Queue: Alex Yang <aycyang@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Alex Yang <aycyang@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1415202}