Updates the toc and makes the data docs
more discoverable.

Currently it's registered after we call into `completeOAuthFlow`, which means
that state mutations that happen as account is logged in aren't persisted.

This is the preferred way to make sure our persisted account state
is up-to-date.
In the future, various parts of FxA (like SendTab) may mutate
account's internal state, and so it's necessary to make sure we
persist account changes as they happen.

Before, this happened manually after account creation.
Now persistence process is entirely callback-driven.

This removes the proguard rules (no longer required) and
adds a link to discover the custom.md file.

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