Support get/set voims_opt_in_status in ImsManager

1. Add getConfigInt() / setConfig() for the config that supported by
framework
2. Return true directly in isVolteEnabledByPlatform() if
KEY_VOIMS_OPT_IN_STATUS has been set.

Bug: 177010143
Test: By Manual
1. Call setProvisioningIntValue() and getProvisioningIntValue()

2. Enable/disable the value and dial a VoLTE call
2-1. Set value to 1
2-2. Dial a call
2-3. From ImsPhoneCallTracker and GsmCdmaPhone's log, VoLTE is
 registered and device is trying IMS PS call
2-4. Set value to 0
2-5. Dial a call
2-6. From ImsPhoneCallTracker and GsmCdmaPhone's log, VoLTE is
unregistered and device is trying (non-IMS) CS call

Change-Id: Ic94a382f301b3652b51fa3b024ce480836fef106
Merged-In: Ic94a382f301b3652b51fa3b024ce480836fef106