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.

Test: atest DataConnectionTest
Test: atest DcTrackerTest
Bug: 185497057
Change-Id: I29d986e6e39f0ed345b72be85a384c9137ca8f2e