Add security level information to BrowserKey.

This change introduces a `SecurityLevel` enum to `BrowserKey` to
indicate whether a key is backed by software, a Trusted Execution
Environment, or StrongBox. The Android implementation queries the Java
KeyStore to determine the security level of the private key. This will
be used for UMA metric for insights.

Bug: 432304139
Change-Id: If28c101679f90383daaca33303655ee67491ec8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7021971
Reviewed-by: Sébastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Elias Homsi <eliashomsi@google.com>
Cr-Commit-Position: refs/heads/main@{#1530259}