Update javadoc of APIs TelephonyManager.is(Device)VoiceCapable

Improve the javadoc of the deprecated API TM.isVoiceCapable, according
the suggestion from API council review:
- Enrich the deprecation message on the background of the deprecation
- Explicitly specify when caller should check the new subscription
  capability API

Bug: 318484199
Test: m -j offline-sdk-docs
Change-Id: Ifbcb6a56cc149ed110fbbdba58445a164c9cb4d2