Replace run-time modes by compile-time constants

Simplify the code to use the constant values of MODE_STATIC and
MODE_STREAM from AudioTrack.java.

There is already a comment in AudioTrack.java saying that these symbols
need to be kept in sync with this file, and they are public static final
so they can never be changed.

Change-Id: I6cb10ac511df4845610454d1659e64d9195d67cd