Add test for AppIconCacheManager to make sure the singleton works properly in multi-threading

package com.android.settingslib.applications;

import static android.content.ComponentCallbacks2.TRIM_MEMORY_BACKGROUND;

import static com.google.common.truth.Truth.assertThat;

import static org.mockito.Mockito.doReturn;

import android.graphics.drawable.Drawable;

import android.util.Log;

import org.junit.After;

import org.junit.Before;

import org.mockito.MockitoAnnotations;

import org.robolectric.RobolectricTestRunner;

import java.util.concurrent.atomic.AtomicInteger;

@RunWith(RobolectricTestRunner.class)

public class AppIconCacheManagerTest {

    private static final String TAG = "AppIconCacheManagerTest";

    private static final String APP_PACKAGE_NAME = "com.test.app";

    private static final String APP_PACKAGE_NAME1 = "com.test.app1";

    private static final String APP_PACKAGE_NAME2 = "com.test.app2";

        assertThat(mAppIconCacheManager.get(APP_PACKAGE_NAME2, APP_UID)).isNull();

        assertThat(mAppIconCacheManager.get(APP_PACKAGE_NAME3, APP_UID)).isNull();

    }

    @Test

    public void trimMemory_multiThread_shouldNotCrash() {

        int numberOfTasks = 10;

        AtomicInteger completedTasks = new AtomicInteger(0);

        Runnable task =

                () -> {

                    String threadName = Thread.currentThread().getName();

                    Log.i(TAG, "Starting thread: " + threadName);

                    AppIconCacheManager.getInstance().trimMemory(TRIM_MEMORY_BACKGROUND);

                    completedTasks.incrementAndGet();

                    Log.i(TAG, "Ending thread: " + threadName);

                };

        for (Integer i = 0; i < numberOfTasks; i++) {

            Thread thread = new Thread(task);

            thread.start();

        }

        while (completedTasks.get() < numberOfTasks) {

            // Wait until all threads are finished.

        }

    }

}