ref(android): Mark AppStartMetrics.setAppStartType as @testonly (#6121)

No production code calls this setter; the cold/warm classification assigns
the field directly. Its only callers are tests, so annotate it the way the
neighboring test seams (setFirstIdle, getFirstIdle, clear) already are.

It has to stay public: most of those call sites live in
io.sentry.android.core, a different package from AppStartMetrics.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: runningcode

sentry-android-core/src/main/java/io/sentry/android/core/performance/AppStartMetrics.java

@@ -156,6 +156,7 @@ public AppStartMetrics() {
     return applicationOnCreate;
   }
 
+  @TestOnly
   public void setAppStartType(final @NotNull AppStartType appStartType) {
     this.appStartType = appStartType;
   }