Add getImei() to Phone interface.

Need IMEI to be displayed in LTE Settings. Since getDeviceId() returns MEID for LTE device,
getImei() is added to return IMEI for all devices. getImei returns null if not set (as SipPhone).

bug:4983512
Change-Id: Idfe2ec8de514f330248db7a9de7e241f9a38cb96