SamsungExynos3RIL: buffer CAT Proactive Command

The RIL sends a CAT proactive command (init) before the registrant
is registered causing the command to be missed and STK menu to
not show up. Buffer it and fire it when the registrant is registered.

Change-Id: I021ce911e55b30c1a2d95d8060f7d9ff72e3b742