Make loading page progress bar accessible to TalkBack.

[eeejay]

ProgressBars can be made accessible to TalkBack by setting android:accessibilityLiveRegion.
They will emit TYPE_VIEW_SELECTED events. TalkBack will format those events into percentage
announcements along with a pitch-change earcon. We are not using that feature here for
several reasons:

1. They are dispatched via a 200ms timeout. Since loading a page can be a short process,
   and since we only update the bar a handful of times, these events often never fire and
   they don't give the user a true sense of the progress.
2. The last 100% event is dispatched after the view is hidden. This prevents the event
   from being fired, so the user never gets a "complete" event.
3. Live regions in TalkBack have their role announced, so the user will hear
   "Progress bar, 25%". For a common feature like page load this is very chatty and unintuitive.
4. We can provide custom strings instead of the less useful percentage utterance, but
   TalkBack will not play an earcon if an event has its own text.

For all those reasons, we are going another route here with a "loading" announcement
when the progress bar first appears along with scroll events that have the same
pitch-change earcon in TalkBack (although they are a bit louder). This gives a concise and
consistent feedback to the user that they can depend on.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[pocmo]

There's one lint issue that should be fixed after #2508 lands.

[codecov]

Codecov Report

Merging #2526 into master will increase coverage by 0.62%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##             master    #2526      +/-   ##
============================================
+ Coverage      82.5%   83.12%   +0.62%     
+ Complexity     2877     2815      -62     
============================================
  Files           354      348       -6     
  Lines         12129    11778     -351     
  Branches       1786     1719      -67     
============================================
- Hits          10007     9791     -216     
+ Misses         1382     1265     -117     
+ Partials        740      722      -18

Impacted Files	Coverage Δ	Complexity Δ
...mponents/browser/toolbar/display/DisplayToolbar.kt	93.08% <81.81%> (-0.18%)	41 <3> (+1)
...lla/components/service/glean/DatetimeMetricType.kt	77.41% <0%> (-16.13%)	19% <0%> (-1%)
...nts/service/glean/firstrun/FileFirstRunDetector.kt	88.23% <0%> (-11.77%)	6% <0%> (ø)
...onents/service/glean/storages/PingStorageEngine.kt	70% <0%> (-6.09%)	14% <0%> (+2%)
...ain/java/mozilla/components/service/glean/Glean.kt	85.88% <0%> (-5.96%)	1% <0%> (ø)
...nents/browser/storage/sync/PlacesHistoryStorage.kt	94.54% <0%> (-3.13%)	19% <0%> (+2%)
...a/components/browser/toolbar/facts/ToolbarFacts.kt	91.66% <0%> (-2.78%)	0% <0%> (ø)
...ents/service/glean/storages/EventsStorageEngine.kt	83.01% <0%> (-0.61%)	15% <0%> (-11%)
...s/browser/storage/memory/InMemoryHistoryStorage.kt	89.28% <0%> (-0.37%)	30% <0%> (-4%)
...a/components/service/glean/net/HttpPingUploader.kt	95.45% <0%> (ø)	9% <0%> (ø)	⬇️
... and 10 more

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[Pike]

Fly-by comment, I see that this patch landed localizations, too. Those don't get synchronized to the l10n process, and won't be seen by the l10n community.

If you're up to contribute localizations for your patches, doing so on Pontoon will be more transparent.