Return mapping of subscription id to unique display name.

Fallback 1: if display names are the same, use the last 4 digits of a
phone number
Fallback 2: if the phone number cannot be displayed or the last 4 digits
are identical, fallback to use the subscription id

This function depends on DeviceInfoUtils for the phone number
formatting. Because DeviceInfoUtils is within SettingsLib, this function
(at least for now) is located within the Settings codebase itself.

go/unique-sub-display-names

Bug: 148303118
Bug: 150370656
Test: atest -c SubscriptionUtilTest
Change-Id: Id3e26254d4b193c13506df52fb59e97643df7a32