Add class comment to avoid extending ServiceState class.

ServiceState implements Parcelable and all 'Parcelables' are required to
be final, per the Android API Guidelines (go/android-api-guidelines).
This won't be changed for historical reasons, but clients should avoid
extending this class.

Bug: 128865231
Test: non-functional change that won't impact tests.
	atest CtsTelephonyTestCases:ServiceStateTest
Change-Id: Ib369f1449a3e085f404fff1c706fda0b76d42890