Add meaningful content descriptions to tracking protection toolbar icon for different states

[Amejia481]

Every time that we change that a tracking protection icon in the toolbar we have to add a meaningful content description to be read by screen readers tools

┆Issue is synchronized with this Jira Task

[Amejia481]

These are the actual strings that we are using:

<!-- Content description: For the tracking protection toolbar icon, it is set when the site has tracking protection enabled, but none trackers have been blocked or detected. -->
    <string name="mozac_browser_toolbar_content_description_tracking_protection_on_no_trackers_blocked">Tracking Protection is on</string>
    <!-- Content description: For the tracking protection toolbar icon, it is set when the site has tracking protection enabled, and trackers have been blocked or detected.-->
    <string name="mozac_browser_toolbar_content_description_tracking_protection_on_trackers_blocked">Tracking Protection is active blocking trackers</string>
    <!-- Content description: For the tracking protection toolbar icon, it is set when the site has tracking protection disabled.-->
    <string name="mozac_browser_toolbar_content_description_tracking_protection_off_for_a_site">Tracking Protection is disabled for this site</string>

cc: @mheubusch

[mheubusch]

Tracking Protection is on
Tracking Protection has blocked trackers
Tracking Protection is off for this site

[Amejia481]

Thanks!