Fall back to default assistant

If an alterante assistant is seleceted and gets uninstalled, we want to
fall back to the default assistant. We still want to support the user
choose to explicitly selecting no assistant at all.

From the system perspective there is no difference between "the user
choose to not select a role holder" and "try to fall back to a role
holder". The permission controller stores the difference between these
choices in a shared preference and checks it when appropriate.

Test: - Selected alternate assistant and removed it -> default assistant
        was chosen
      - Selected no assistant -> settings was honored
      - Selected no assistant and triggered grantDefaultRoles -> no
        change
Bug: 110557011
Change-Id: I8b0c541e2d2483f3232e3cbe86caefb3a7bf600c