Add API to enable/disable the visual voicemail client

This CL gives the default dialer, apps with carrier privilege, and
apps with MODIFY_PHONE_STATE the ability to enable/disable the visual
voicemail client.

This is a system API for NMR1. It will be revised in O.

Bug: 30709089
Change-Id: Ic24af14a5e3433c13ba8d253ad8504c94bf71c8f
3 files changed