Apply @hide / @SystemApi to android.telecom.*

Move the android.telecom.* namespace back to @hide, and also mark it
with @SystemApi so that system-privileged apps can use them.

Bug: 18302450

Conflicts:
	telecomm/java/android/telecom/Conference.java
	telecomm/java/android/telecom/Connection.java
	telecomm/java/android/telecom/PhoneAccount.java
	telecomm/java/android/telecom/TelecomManager.java

Change-Id: I33ae1b9b0dfdb1c5eff51ca3c829196bcfc9411c