[components] Use new geckoview-omni package, which includes Glean directly
The native Glean core code is now shipped as part of GeckoView, within
the `-omni` packages.
The Glean Gradle plugin takes care that this all fits together nicely.
Downstream users will need to depend on the `glean-native` package for
tests now.
For now Android Components still needs to declare an explicit dependency
on `org.mozilla.telemetry:glean` as well.
service-glean needs to know what to reexport and some other components
depend on it directly to be able to use it. These might not depend on
geckoview, so have no other way to get that dependency.
Now here's the catch: Because GeckoView defines `org.mozilla.telemetry:glean` as a dependency,
consumers that transitively depend on GeckoView (through
`browser-engine-gecko`) will get the latest version of `org.mozilla.telemetry:glean` through it.
Gradle unifies the versions.
It would be problematic if a-c depends on a higher version, but that
should not happen (though we don't have any technical limitations on it
yet).
If Glean does a breaking change release and gets that into
mozilla-central, the version in a-c needs to be manually updated too.
For other releases it can be left untouched.