address carrier id api review feedback

1. improve comments
2. change returned carrier name from String to CharSequence
3. change to AndroidCarrierId

Bug: 64131637
Bug: 71584605
Test: Manual
Change-Id: I412d53bb9d4955e1267300692e90ebfffd6818df
Merged-in: I412d53bb9d4955e1267300692e90ebfffd6818df
2 files changed