Add build variant for go-specific recents.

This CL sets up the source directories to build a Go version of
quickstep recents/icon recents. This is accomplished by splitting the
quickstep resources/sources into two groups: the base source and
sources that should be overridden (recents_ui_overrides). The new Go
recents variant then takes the base and combines it with the sources
in go/quickstep which should replace the ones in recents_ui_overrides.

As of now, no classes have been moved yet, so there
are only placeholder files.

In addition, this modifies the make and gradle files to reflect the new
sources.

Bug: 114136250
Test: build all targets via tapas, build all APKs from IDE
Change-Id: I6d0a4d93ab1c38d4ffc28fd21823caadd96d23f7