Replace run-time PCM encodings by compile-time constants

Simplify the code to use the constant values of ENCODING_PCM_16BIT and
ENCODING_PCM_8BIT from AudioFormat.java.

There is already a comment in AudioFormat.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.

This change is similar to commit 3d301cb2
or Change-Id I6cb10ac511df4845610454d1659e64d9195d67cd

Change-Id: Ib62d8cfeb6261ffd362dba3600d593e2a268d76a