app_shim_manager_mac: use the framework's static code signing info

* Move away from using SecCodeCopySelf() to get the signing info base for validating app shims. Instead use SecStaticCodeCreateWithPath() to pull the live framework's signing info.
* This CL also makes app shim validation less brittle.

Bug: 1263152, 1281111
Change-Id: If0d57f9e3b1d410d618cf62bbd2da78186ce03d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3453293
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Tom Burgin <bur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#971408}