Set Opportunistic as data-centric

When a usage setting is set to "default", if the
subscription is opportunistic, it should be set to
data-centric to preserve backwards compatibility
with the assumption that all opportunistic subscriptions
are data-centric.

Bug: 211689395
Test: atest GsmCdmaPhoneTest#testUsageSettingUpdate_DataCentric
Test: atest GsmCdmaPhoneTest#testUsageSettingUpdate_DefaultOpportunistic
Test: atest GsmCdmaPhoneTest#testUsageSettingUpdate_DefaultNonOpportunistic
Change-Id: I3f996621ef18292598965708144a2b3ac70f3e58