I'm in the process of porting the java-stellar-sdk library (found at https://github.com/stellar/java-stellar-sdk) to be a drop-in Android dependency through gradle. The current java library has a few problems:
1) It compiles to a .jar file. Though this theoretically works, it is unusual for an Android dependency to be manually added through the /libs folder. It's much easier (and more conventional) for the library to be added as a gradle dependency and pulled down automatically through jCenter or mavenCentral.
2) HttpClient was deprecated in API level 22 and removed in API level 23. (https://stackoverflow.com/questions/32153318/httpclient-wont-import-in-android-studio and https://developer.android.com/about/versions/marshmallow/android-6.0-changes.html#behavior-apache-http-client) Because of this, it's extremely difficult to get any of the API calls from the SDK working in an Android app. I am converting the networking layer to use OkHttp (https://square.github.io/okhttp/) which is the most popular and widely supported Android HTTP client. This will also allow for future optimizations like adopting Retrofit, if so desired.
3) Some other small classes used here and there in the java SDK have also been removed from Android including some of the Base64 conversion classes.
I have done the majority of the work required for this conversion and am currently updating the tests to reflect the OkHttp changes. I plan to be finished by early-mid Feb. I'm confident this library will make it significantly easier for Android developers to create really cool applications for Stellar!