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

No production code calls this setter; the cold/warm classification assigns
the field directly. Its only callers are the 45 test sites that seed a start
type, so annotate it the way the neighboring test seams (setFirstIdle,
getFirstIdle, clear) already are.

It has to stay public: 37 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 +156,11 @@ public AppStartMetrics() {
     return applicationOnCreate;
   }
 
+  /**
+   * Seeds the start type directly, bypassing the cold/warm classification. Production code assigns
+   * {@link #appStartType} as part of that classification instead.
+   */
+  @TestOnly
   public void setAppStartType(final @NotNull AppStartType appStartType) {
     this.appStartType = appStartType;
   }