Address API Council Feedback for NetworkServiceState APIs

This changes:
(1) Updates getOperatorName to getNetworkOperatorName. This is in line
with both the HFP specs wording and the related TelephonyManager API.
(2) Adds an IntRange annotation to the getSignalStrength. This
annotation was chosen because the spec doesn't have definitive wording
on how the 0 to 5 numbers are derived, despite the fact that we
generally treat them as (None/Unknown, Very Poor, Poor, Moderate, Good,
Great). IntRange adds some clarity around the domain of values without
making any claims about the exact meanings.

Tag: #refactor
Bug: 219820175
Ignore-AOSP-First: Mainline related API Council Feedback change
Test: atest BluetoothInstrumentationTests
Change-Id: I5974bc4ee39113a3ebbffaacd28c0d5af70483ad