Some good ANT+ cycling sensors that are compatible with CycleBike+ and Bluetooth sensors for CycleBike Smart.
Step-by-step instructions for developing apps for Android. Take their Java class first. The courses for Android are fast-paced.
There's a learning curve, especially Gradle, but this IDE is easier to use than Eclipse to develop for Android.
The definitive source for documentation on Android. Stack Overflow has some tips (usually hacks if it's older than 2012), but go straight to the real documentation here.
Android apps can be graphic-intensive. GIMP has some good tools, but there's a learning curve. Create a path first before drawing. Use lots of layers.