Привет!
Я бы рекомендовал изучить темы:
Java
--------
* Object: hashCode, equals, wait, notify
* Collections: List, Set, Queue, Map
* References: Strong, Soft, Weak, Phantom
* Concurrency: Thread, Runnable, Executor, Future, Synchronized, Volatile, Synchronized collections.
* io: InputStream, OutputStream, File, serialization
* Network: URL, urlconnection, etc
* json, xml
* generics
* reflection
* class loader
* enum
Android
--------
* Android tasks and backstack (LaunchMode(singleTask, singleTop, etc), Task Affinity)
* Activity: Lifecycle, What, start, start with result
* Services: IBinder, Message Handler
* ContentProvider: how to use and why
* Broadcast Receiver: how to use and why
* Intent & Intent Filters: common and in manifest.xml
* Fragments: Lifecycle, Why, FragmentManager, NestedFragmentManager
* Notifications: how to use
* Permissions: what and how
* Resources: what and how
* Styles: how to use and why
* InputEvents
* MaterialDesign: RecyclerView, CardView, FloatingActionButton
* Layouts (also CoordinatorLayout)
* Toolbar
* Pager
* Settings
* Menu
* Dialogs
* Toasts, Snackbar
* Animation, Shadows
* Support lib, Support design
* Network: frameworks (OkHttp, Retrofit)
* Process: separated processes
* Application: using
* Concurrency: AsyncTask, best practices for long time work.
* Bitmaps: using, reusing
* VectorDrawable
* Deep Linking
Common:
--------
* Patterns
* Caching
* di
Misc:
-------
Git
Gradle