From the AOSP Java Code Style for Contributors:

Follow Field Naming Conventions

• Non-public, non-static field names start with m.
• Static field names start with s.
• Other fields start with a lower case letter.
• Public static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.

For example:

public class MyClass {
    public static final int SOME_CONSTANT = 42;
    public int publicField;
    private static MyClass sSingleton;
    int mPackagePrivate;
    private int mPrivate;
    protected int mProtected;
}

Our developer survey shows that we overall like the "identifier metadata" and will stick to this convention.

We don't believe that "Today, our IDE’s can distinguish all these identifiers with colors so we don’t need any of this any more" because we often read code outside of IDEs when reviewing code.

Cédric Beust outlined his decision to introduce the m-prefix in his article "I am the reason for Hungarian notation in Android". Read this for further arguments for the m-prefix.