Check out this section from developer.android.com:
In native languages like C++ it's common practice to use getters (i = getCount()) instead of accessing the field directly (i = mCount). This is an excellent habit for C++ and is often practiced in other object oriented languages like C# and Java, because the compiler can usually inline the access, and if you need to restrict or debug field access you can add the code at any time.
However, this is a bad idea on Android. Virtual method calls are expensive, much more so than instance field lookups. It's reasonable to follow common object-oriented programming practices and have getters and setters in the public interface, but within a class you should always access fields directly.
Without a JIT, direct field access is about 3x faster than invoking a trivial getter. With the JIT (where direct field access is as cheap as accessing a local), direct field access is about 7x faster than invoking a trivial getter.
Note that if you're using ProGuard, you can have the best of both worlds because ProGuard can inline accessors for you
http://developer.android.com/training/articles/perf-tips.html#GettersSetters
Most likely, this is the reason you see public fields in AOSP.