Make SubscriptionManager.getSlotIndex public

- Make public
- Improved the documentation
- Rename subId to subscriptionId
- Make INVALID_SIM_SLOT_INDEX, SIM_NOT_INSERTED public

Test: Treehugger
Bug: 113072551
Change-Id: Idaf5ed3672be421568c3618cd94534c27f64f64c
2 files changed