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()`.

This is:

1. converting string classifier tests in extension methods
2. changing to use `WebURLFinder` in the `IntentProcessor`.

Glean pings are currently directly written to their final
location. Some other thread might be scanning that location
for uploading pings and might see partially written files.
This PR writes pings to the cache directory and, after they
are fully written, moves them to their final destination.