* For #3869: Switch to A-C SearchLocalizationProvider

* Fix test

RequestDesktopStateProvider was taken from private instead of desktopMode

* Remove "abi" product flavor and introduce "engine" product flavor.

This patch will allow us to build Fenix against GeckoView Nightly and GeckoView Beta by
introducing a new flavor dimension: engine = [geckoNightly, geckoBeta].

In addition to that it adds a "fenix"  prefix to the nightly, beta and production flavors
to reduce the ambiguity between fenix beta/nightly and GeckoView beta/nightly.

For now the build types have the following engine variants enabled:

**debug**: geckoNightly, geckoBeta
Both variants enabled for local development and testing.

**forPerformanceTest**: geckoNightly, geckoBeta
Both variants enabled unless the perf team only cares about Nightly (tbd)

**fenixNightlyLegacy**: geckoBeta
Uses GeckoView Beta for now - the same version we ship production builds with (same behavior
as before). This release type will eventualyl be decommissioned once we switch to a separate
Nightly app on Google Play.

**fenixNightly**: geckoBeta
Uses GeckoView Beta for now - the same version we ship production builds with (same behavior
as before). Changing this build to use GeckoView Nightly is currently being discussed.

**fenixBeta**: geckoBeta
Fenix Beta uses GeckoView Beta.

**fenixProduction**
Fenix Production uses GeckoView Beta (69) currently.

* gradle.py/variant.py: Replace "abi" with "engine".

* Disable enableUnitTestBinaryResources until we can switch to Android Gradle plugin 3.5.

* Fenix nightly should use both geckoview nightly and beta

* Updates automation to use apk splitting and support different engine

* Change Settings to use setters

* Add delegate for shared preferences

* For #4399: Create LibState Controller for Search

* fix code format

* add unit tests for DefaultSearchController

* add more test

* fix unit tests

New tests for `FragmentPreDrawManager` are to be added in #4828

The previous solution would result in a crash because the passed in
viewTreeObserver that would trigger onPreDraw would be invalid.
The proposed solution is simpler and ensures we'll always use the right
viewTreeObserver.

`FragmentPreDrawManager` is general enough that can be used by other Fragments
also, so I've added it to the `utils` package.

Remove space in placeholder for es (temp for release)

State: mozilla-l10n/android-l10n@413e05dba427ef8cbf9e5272ae5cc3b3ccdf91d3