arc: Add ShouldUseKeyMint() as utility function

ShouldUseKeyMint() is a non-temporary function that decides whether
KeyMint should be used.

When KeyMint is fully rolled out on ARC-T, the return value will stop
depending on the value of feature "ArcSwitchToKeyMintOnT".

Bug: b:247941366
Test: autoninja -C out_hatch-arc-t/Release/ chrome chrome_sandbox
nacl_helper

Change-Id: Ia9858be10d89ff932edd341960aa4ef7a9eff772
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4635277


Reviewed-by: Josh Horwich <jhorwich@chromium.org>
Commit-Queue: Yao Li <yaohuali@google.com>
Cr-Commit-Position: refs/heads/main@{#1162687}