[SafetyCheck] Split up notification permissions feature flag in two

In SafetyCheck, unused site permissions and notification permissions
currently are one feature and controlled by a single flag. To enable
separate testing and rollout they are split up in two different
features and flags.

This is the continuation of http://crrev.com/c/3898537 which can no
longer be edited due to permission issues with chromium account
linking in gerrit.

Bug: 1345920
Change-Id: I15cacabc33c7b4ef754b9ff2838292ca80e289c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3904265
Reviewed-by: Rainhard Findling <rainhard@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Side YILMAZ <sideyilmaz@chromium.org>
Commit-Queue: Eduard Hez <ehez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1050138}