There are a lot of topics that need be covered when learning Android development.

Beginner

• Buttons / Event Handlers / Event Listeners / Selectors
• Activities and Activity Life Cycles
• Intents and Intent Filters
• Strings and String Resources
• XML and Java code
• Basic Views (TextView, ImageView) and View Groups
• Layouts (Relative, Linear, Table, Grid)
• Other Components (LargeText)
• Log, Toasts, SnackBar

Intermediate

• Dynamic Views (ListViews / GridViews / RecyclerView)
• Adaptors and Inflators
• Navigation (Menu, ActionBar/ToolBar, BackStack, SlideDrawer)
• Fragments
• Graphics (Animations, Material Design, Custom Drawing of Views)
• Multi-screen and orientation
• WebView
• Storage (XML / JSON)
• Google Store (Submitting, Integrating Ad)
• Media (Audio / Video)
• Form elements (Radio buttons / Checkboxes)
• Model View Controller (MVC)

Advanced

• Networking (JSON, HTTP requests, WebView)
• Data Storage (SQLite, Shared Preferences)
• Database and SQLite3 (DBMS)
• APIs Sensors (Accelerators, GPS, Gyroscope, camera, Bluetooth, geolocation)
• Services
• Content Provider
• Notifications
• Broadcast Receiver (battery level / email notification)