Inject firebase common executors to Crashlytics (#6019)

According to the documentation
[here](https://firebase.github.io/firebase-android-sdk/components/executors/).
Inject Firebase common executors for later refactor work.

Crashlytics will need at least 2 types of executors: background(for disk
I/O writes), blocking(networking calls).

Note: all changes later will also merge to the feature branch:
`crashlytics-exception-handler`

Author: themiswang

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/CrashlyticsRegistrar.java

@@ -16,10 +16,13 @@
 
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.analytics.connector.AnalyticsConnector;
+import com.google.firebase.annotations.concurrent.Background;
+import com.google.firebase.annotations.concurrent.Blocking;
 import com.google.firebase.components.Component;
 import com.google.firebase.components.ComponentContainer;
 import com.google.firebase.components.ComponentRegistrar;
 import com.google.firebase.components.Dependency;
+import com.google.firebase.components.Qualified;
 import com.google.firebase.crashlytics.internal.CrashlyticsNativeComponent;
 import com.google.firebase.inject.Deferred;
 import com.google.firebase.installations.FirebaseInstallationsApi;
@@ -29,10 +32,14 @@
 import com.google.firebase.sessions.api.SessionSubscriber;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.ExecutorService;
 
 /** @hide */
 public class CrashlyticsRegistrar implements ComponentRegistrar {
   private static final String LIBRARY_NAME = "fire-cls";
+  private final Qualified<ExecutorService> backgroundExecutorService = Qualified.qualified(Background.class, ExecutorService.class);
+  private final Qualified<ExecutorService> blockingExecutorService = Qualified.qualified(Blocking.class, ExecutorService.class);
+
 
   static {
     // Add Crashlytics as a dependency of Sessions when this class is loaded into memory.
@@ -46,6 +53,8 @@ public List<Component<?>> getComponents() {
             .name(LIBRARY_NAME)
             .add(Dependency.required(FirebaseApp.class))
             .add(Dependency.required(FirebaseInstallationsApi.class))
+            .add(Dependency.required(backgroundExecutorService))
+            .add(Dependency.required(blockingExecutorService))
             .add(Dependency.deferred(CrashlyticsNativeComponent.class))
             .add(Dependency.deferred(AnalyticsConnector.class))
             .add(Dependency.deferred(FirebaseRemoteConfigInterop.class))
@@ -70,6 +79,6 @@ private FirebaseCrashlytics buildCrashlytics(ComponentContainer container) {
         container.getDeferred(FirebaseRemoteConfigInterop.class);
 
     return FirebaseCrashlytics.init(
-        app, firebaseInstallations, nativeComponent, analyticsConnector, remoteConfigInterop);
+        app, firebaseInstallations, nativeComponent, analyticsConnector, remoteConfigInterop, container.get(backgroundExecutorService),  container.get(blockingExecutorService));
   }
 }

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/FirebaseCrashlytics.java

@@ -24,6 +24,8 @@
 import com.google.android.gms.tasks.Tasks;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.analytics.connector.AnalyticsConnector;
+import com.google.firebase.annotations.concurrent.Background;
+import com.google.firebase.annotations.concurrent.Blocking;
 import com.google.firebase.crashlytics.internal.CrashlyticsNativeComponent;
 import com.google.firebase.crashlytics.internal.CrashlyticsNativeComponentDeferredProxy;
 import com.google.firebase.crashlytics.internal.DevelopmentPlatformProvider;
@@ -68,7 +70,9 @@ public class FirebaseCrashlytics {
       @NonNull FirebaseInstallationsApi firebaseInstallationsApi,
       @NonNull Deferred<CrashlyticsNativeComponent> nativeComponent,
       @NonNull Deferred<AnalyticsConnector> analyticsConnector,
-      @NonNull Deferred<FirebaseRemoteConfigInterop> remoteConfigInteropDeferred) {
+      @NonNull Deferred<FirebaseRemoteConfigInterop> remoteConfigInteropDeferred,
+      @Background ExecutorService liteExecutorService,
+      @Blocking ExecutorService blockingExecutorService) {
 
     Context context = app.getApplicationContext();
     final String appIdentifier = context.getPackageName();