Atom: CallStateChanged

Adds CallStateChanged atom, including the state and disconnect cause
(when the state is disconnected). Makes the constants in CallState.java
(in packages/services) and DisconnectCause.java reference the proto
enum.

Test: atest TelecomUnitTests, verified events were received in adb logcat -b stats
Change-Id: I5c4d91387259fc7349e17551962cb490f5ed9f36