[HFM] Typically Secure heuristic enables balanced mode

This CL modifies the Typically Secure heuristic to enable HTTPS-First
Balanced Mode instead of Strict Mode, when possible. Previously, users
with typically secure browsing habits (i.e. very few navigations to
sites that don't support HTTPS over a rolling window) would
automatically have HTTPS-First Mode enabled. Balanced Mode is a more
lenient version of HFM, and is more suitable for automatically enabling,
if possible.

This CL makes the following changes:
1. Moves the feature flag check
2. Only enables Balanced Mode if it's available via the feature flag

Bug: 40925331, 371000390
Change-Id: I0b17c47c57d40806c17c32aab15d064c7bd3ea5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5891528
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1364356}