Remove code related to stolen trace sessions

As of aosp/2323319, tracing_service_proxy will always report
session_stolen=false. This change updates TracingServiceProxy.java to
reflect this.

Test: Check that System Tracing + bug report interactions are still
      correct.
Bug: 352779024
Flag: EXEMPT no-op cleanup
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1652699005326167786d0d1e68625a974e268a18)
Merged-In: I9b199c3040db47cdf1c6ef7cd03567e785a63b17
Change-Id: I9b199c3040db47cdf1c6ef7cd03567e785a63b17