This shifts responsibility of constructing a VisitType for any given visit on the
caller of HistoryTrackingDelegate implementation, which is normally the engine.

This is generally what we want: engine is normally best suited to determine a visit type
for a URI that's currently being opened.

Status: mozilla-l10n/android-l10n@6f5b72b05bb7e250f72f0c931565c38e2b490714

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.