Update target sdk version to 28

- LEARNER-7398

Context: PlayStore no longer allows uploading apps to it that have
targetSdkVersion lower than 28. Following are the updated guidelines:

- New apps must target at least Android 9.0 (API level 28)
- From November 1, 2019, app updates must target at least Android 9.0.
  Until this date, app updates must target at least Android 8.0
  (API level 26).

More info on migration and its checklist:
https://developer.android.com/about/versions/pie/android-9.0-migration

constants.gradle

@@ -5,7 +5,7 @@ project.ext {
     SUPPORT_LIBRARY_VERSION = "28.0.0"
     COMPILE_SDK_VERSION = 28
     // API Level that the application will target
-    TARGET_SDK_VERSION = 27
+    TARGET_SDK_VERSION = 28
     // Minimum API Level required for the application to run
     MIN_SDK_VERSION = 19