refactor(sentry): Use custom CompletedFuture for API 21+ compatibility (EME-413)

Replace CompletableFuture.completedFuture() with a custom CompletedFuture
implementation in NoOpDistributionApi to maintain Android API 21+ compatibility.
CompletableFuture was only added in Android API 24.

The new CompletedFuture follows the same pattern as existing CancelledFuture
implementations in the codebase and returns an immediately completed Future
with a result.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: runningcode

sentry/src/main/java/io/sentry/NoOpDistributionApi.java

@@ -1,7 +1,9 @@
 package io.sentry;
 
-import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
 
@@ -24,11 +26,49 @@ public static NoOpDistributionApi getInstance() {
 
   @Override
   public @NotNull Future<UpdateStatus> checkForUpdate() {
-    return CompletableFuture.completedFuture(UpdateStatus.UpToDate.getInstance());
+    return new CompletedFuture<>(UpdateStatus.UpToDate.getInstance());
   }
 
   @Override
   public void downloadUpdate(@NotNull UpdateInfo info) {
     // No-op implementation - do nothing
   }
+
+  /**
+   * A Future implementation that is already completed with a result. This is used instead of
+   * CompletableFuture.completedFuture() to maintain compatibility with Android API 21+.
+   */
+  private static final class CompletedFuture<T> implements Future<T> {
+    private final T result;
+
+    CompletedFuture(T result) {
+      this.result = result;
+    }
+
+    @Override
+    public boolean cancel(final boolean mayInterruptIfRunning) {
+      return false;
+    }
+
+    @Override
+    public boolean isCancelled() {
+      return false;
+    }
+
+    @Override
+    public boolean isDone() {
+      return true;
+    }
+
+    @Override
+    public T get() throws ExecutionException {
+      return result;
+    }
+
+    @Override
+    public T get(final long timeout, final @NotNull TimeUnit unit)
+        throws ExecutionException, TimeoutException {
+      return result;
+    }
+  }
 }