Change MobileNetworkUtils to use EuiccManager.isSupportedCountry

Currently LPA passed supported/unsupported countries to Settings by
writing the value into properties which is not ideal. Instead, we should
call EuiccManager.isSupportedCountry instead.
Bug: 147674689
Test: 1) Manually tested by flashing the change to the device
2) Change LPA to override onGetIsEuiccSupportedCountry
3) Make sure LPA can get the request

Change-Id: Ib136beea325eabdfbd8a6a843611143958dce603