Change osAppId from String to byte[] to match specs.

OSAppId as defined in the specs is a concatenation of a UUID
(representing OSId) converted to byte[], and a byte[] representing
the app id. As such it is better to have that as a byte[] in the
public APIs instead of String.

Made the Builder @hide as well since it is not supposed to be
built by any external code.

Test: atest DataConnectionTest
Test: atest DcTrackerTest
Test: atest TrafficDescriptorTest
Bug: 185497057
Change-Id: I3d327097397bcfaaca2e6b7c342fb440fd546318