[Sb refactor] Filter out -1 values for ERI

TelephonyManager.getCdmaEnhancedRoamingIndicatorNumber will return -1
before the service is available. Filter these out so we don't
accidentally consider the connection to be `cdmaRoaming` when actually
we don't have that information yet.

Test: MobileConnectionRepositoryTest
Bug: 269040632
Change-Id: I5d7762b7a671ef1ab48130eb051abcba04c231eb