DeclarativeNetRequest: Add DeclarativeNetRequestPrefsHelper

This CL is a part of adding these two APIs:
- DeclarativeNetRequest.updateStaticRules()
- DeclarativeNetRequest.getDisabledRuleIds()

Add DeclarativeNetRequestPrefsHelper class that provides setter and
getter of the disabled static rule ids.
- GetDisabledStaticRuleIds(<extension_id>, <ruleset_id>)
- GetDisabledStaticRuleCount(<extension_id>)
- UpdateDisabledStaticRules(<extension_id>, <ruleset_id>,
                            <rule_ids_to_update>)

The disabled rule ids of static rulesets are stored in the extension
prefs as a dictionary:
- key: kDNRStaticRulesetPref + kDNRDisabledStaticRuleIds
- value: { <ruleset id> : <disabled rule ids> }

Bug: 1225229
Change-Id: I58a38df789558fcebaa7633f5e9b12d48eec512b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3999643
Commit-Queue: Byungwoo Lee <blee@igalia.com>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1080805}