[MEP] UnsupportedOperationException is thrown when carrier app tries to download subscription.

Starting with android T, old onDownloadSubscription API is deprecated and new API with portIndex is introduced. As latest LPA app is not released with new API changes, exception is occurred.
Added condition to fallback to deprecated API in case if the LPA is not implemented the new API.

Bug: 222257028
Test: manually tested with Google Fi activation on sunfish device
Change-Id: Ifc0b828f0f60d456066386957d4bd1080bdd6ae9