prompts.

The Experiments library should update it's experiments list from Kinto automatically & async on initialization.

This requires some larger changes, specifically:
- Introduced `ExperimentsUpdater`, which intends to contain the experiments update logic & scheduling.
- It does not contain periodic updating of experiments yet (to reduce the scope here, i broke this out to [this bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1541973)).
- Added Experiments integration & initialization to the Glean sample app. This allows us testing app integration.
- Require `Glean` to be initialized first, before Experiments.
- On library init (`Experiments.initialize()`)
- automatically load experiments from storage
- automatically start async updating of the experiments list from the server (from Kinto, triggered by `ExperimentsUpdater`)
- Restructure the testing/mocking approach & clean up `ExperimentsTest.kt` significantly.
- Most importantly, make `ExperimentsInternalAPI` testable through instances & don't use the `Experiments` singleton in tests.
- Set up standard mocks that reduce the setup code for most of the tests in `ExperimentsTest.kt`.
- Make `ExperimentsUpdater` waitable on it's async update task for tests.
- Cleanups:
- Delete the alternative `JobService`-based scheduler implementation.
- Flatten the `ExperimentSource` and `ExperimentStorage` hierarchies. They just have one implementation now and are not configurable from library users.
- Move the `evenDistribution()` test over to a more fitting test file.
- Do a first pass on `README.md`, mostly removing parts that don't apply anymore.

State: mozilla-l10n/android-l10n@932938a1cae1ba6f84f79723dd7980486fef223f

Issue #2487: update_geckoview.sh: Add support for updating all GeckoView channels (nightly, beta, release).

We don't expect consuming applications to change some
of the options we expose. A few of them are only meant
to be changed by the `GleanDebugActivity` (internally)
and some of them should only be changed by tests. This
PR makes the primary constructor of the `Configuration`
object internal and introduces a secondary, public
constructor that only exposes things that can be changed.

Also adds better timing distribution deserializer tests

This reduces the overhead of having to do string interpolation from Int to String every time `accumulate` is called and only has to convert this to a map with String keys when serializing.

State: mozilla-l10n/android-l10n@7724298c8ca526f41c782c5e8ab83342bbf02197

- Fixes and issue with `test sending of background pings` that was causing it to timeout when `triggerWorkManager()` was called due to only enqueueing a single response when we actually need two, one for events ping and one for baseline ping.
- Adds an assert to `triggerWorkManager()` around the `PingUploadWorker.uploadPings()` function to help catch similar issues in the future 

This just updates glean_parser to get the improvements from
https://github.com/mozilla/glean_parser/pull/53

- Make serialization and enqueuing of Worker happen in background task.
- Fix race condition in test: `process removes files that do not have UUID names`() was using a background thread in order to write a bad ping, since this is a test and the test was the one doing the writing, it should be synchronously writing instead.
- Add enableTestMode to MetricsPingSchedulerTest