Modify build system

Bug 40083: Make locale ordering in BuildConfig deterministic

Bug 40042: Add option do overwrite timestamp in extension version

Bug 40059: Use MOZ_BUILD_DATE for versionCode

At the same time we adapt MOZ_BUILD_DATE to our needs where it is
actually used and not in tor-browser-build. This gives us more
flexibility. See: tor-browser-build#40084.

Bug 40067: Fix reproducibility issue in classes2.dex

We make sure our MOZ_BUILD_DATE gets used as a source for showing date
related information on the Fenix about page.

Bug 40071: Show only supported locales

Bug 40064: Use Gecko Beta for Nightly and Debug variants

Bug 40123: Allow building the instrumented tests apks for variants other than debug

This allows to specify the variant of the instrumented tests via
a `testBuildType` gradle argument. It also applies a workaround for
a R8 issue from https://issuetracker.google.com/issues/140851070.

Bug 40143: Use deterministic date in Test apk

The build config was using Date() when generating the Test apk's
versionName.