Migrate policy value accessors to typesafe versions.

This covers files under chrome/browser/ash/*.

Reasons for using "unsafe" over type-checking versions:
- Handling multiple policies and types
- Need to differentiate between nullptr (unset) and type mismatch

Bug: 1294817, b/221918700
Change-Id: I60d41a0bf999bb828a51c89ca0f4a066a39656e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3523421
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Igor Ruvinov <igorruvinov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#981105}