Fixes #2673.

This PR protects against the initial `NullPointerException` ever happening.

This is a rare case, and we do not have anything to go on at this point, so we fallback to a new intent, and the user is routed to the home activity.

This changes glean internals so that recording right before
a call to `sendPings` puts the data in that ping. In order
to sanely allow this, `sendPings` now returns a Deferred
which can be used to check if the ping was correctly taken
care of.
This additionally introduces a function to wait for WorkManager
jobs to be queued (in tests) and an `async` facility in Dispatchers.kt

This adds the first command, "updateExperiments" that will force the ExperimentsUpdater to update immediately.

- Adds doc section to README.md
- Adds export statement to AndroidManifest.xml
- Init Logger with a TAG more in line with how the rest of the component uses Logger
- Fix an issue that was preventing the debug activity from working correctly since it was attempting to do networking on main thread.  Had to use a coroutine to perform the update in the background.

Use WorkManager along with a PeriodicWorkRequest to update experiments every 6 hours.

This contains:
- The new JSON file attributes (per [the spec](https://docs.google.com/document/d/1C1k68VHKEhl1fp2oA_kkn23NZxsaloIIAJiukVQKgf0/)).
- Multi-branching support. If a client matches an experiment, they get assigned a random branch.
- Only one experiment is active at a time. That simplifies a bunch of assumptions. Introduced `ActiveExperiment` to keep that state.
- Persistence of active experiment & branch. They are stored/restored via shared prefs, so the assignment and branch stay stable.
- `withExperiment()` was updated to pass the branch name to outside consumers.
- Temporarily disabled one signing test. I'm taking this in this bug, but i need to sort the kinto setup first.
- Refactored library code and tests quite a bit. Among other things aggressive use of named properties for less error prone passing around of data. Also core data classes like `Experiment` now have all arguments required to statically make sure i'm not missing any. Test utilities loosen this up a bit, e.g. `createDefaultExperiment()`.