Replace kFlexBehaviorKey in BoxLayout with kBoxLayoutFlexKey.

Replace kFlexBehaviorKey with kBoxLayoutFlexKey dedicated to BoxLayout
semantics.

Currently BoxLayout only uses the weight attribute of kFlexBehaviorKey.
This is to make the Builder<T> API more structured, but it makes it
difficult for us to refactor when we want BoxLayout to act as a wrapper
for FlexLayout.

Bug: None
Change-Id: I79d3fe075b0af99c56e82972a131fc5322db0806
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5119972
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: wd l <weidongliu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1239508}