Add QuickstepProcessInitializer back to common src

While removing unused files in Go version of quickstep, I removed
QuickstepProcessInitializer which seemed unused but is actually
initialized as a resource based ovverride. This CL puts it back as
common src so that both Launcher3GoIconRecents and the rest of the
variants all use this class.

Bug: 135472635
Test: Run Launcher3GoIconRecents => no exception
Test: Run Launcher3 => no exception
Change-Id: I56f319bac536ec4da96285fab932fe574afa4520