Bump minSdkVersion to 21, and remove MultidexApplication (see description)

For multidex in api 21 and higher, extending from MultidexApplication is not required. Check: https://developer.android.com/studio/build/multidex#mdex-on-l

app/build.gradle

@@ -7,7 +7,7 @@ android {
     defaultConfig {
         applicationId 'me.austinhuang.instagrabbr'
 
-        minSdkVersion 16
+        minSdkVersion 21
         targetSdkVersion 29
 
         versionCode 51

app/src/main/java/awais/instagrabber/InstaGrabberApplication.java

@@ -1,11 +1,10 @@
 package awais.instagrabber;
 
+import android.app.Application;
 import android.content.ClipboardManager;
 import android.content.Context;
 import android.util.Log;
 
-import androidx.multidex.MultiDexApplication;
-
 import com.facebook.drawee.backends.pipeline.Fresco;
 import com.facebook.imagepipeline.core.ImagePipelineConfig;
 
@@ -27,7 +26,7 @@ import static awais.instagrabber.utils.Utils.datetimeParser;
 import static awais.instagrabber.utils.Utils.logCollector;
 import static awais.instagrabber.utils.Utils.settingsHelper;
 
-public final class InstaGrabberApplication extends MultiDexApplication {
+public final class InstaGrabberApplication extends Application {
     private static final String TAG = "InstaGrabberApplication";
 
     @Override