Add comments according to the API review process.

 * Explain what developers should use instead in the @deprecated note for Telephony#BEARER
 * Please add more documentation for network type bitmask - reference to just TelephonyManager is not sufficient because it has so much stuff in it.
 * Also remove the @hide tag for private constants in ServiceState.

Bug: 73751228
Test: Basic telephony sanity
Change-Id: Ie7405a961db4494104750bf406f20b1b4fb8191a