Workaround IPHONE problem

When we send LL_ENC_REQ to iphone early, iphone didn't send
LL_ENC_RESPONSE back to us, later the iphone will disconnect from us.
The workaround is to send LL_ENC_REQ after receive first ATT read request
from iphone, Iphone will be ready at that time by experiment.

bug:13417692
Change-Id: Ic42dda36b927df260113771152cff67032b22e5f