power.stats: Specifying opaqueness of some members

subsysName, railName, powerEntityName, and powerEntityStateName are all
opaque to the Android framework. Emphasizing the opaque nature of these
members in the HAL definition will help developers more quickly
understand the intent of the interface.

Bug: 125380339
Test: make
Change-Id: I42ed1f3cc928726ae146b6be849947b631ae48e6