Fix AAudioLoader

Fixed the following issues:

 -- using a static member variable for a singleton is not thread-safe
    without explicit synchronization. A better approach is to use
    a static variable in 'getInstance' function, which is guaranteed
    to be synchronized by the standard;

 -- the constructor has to be private;

 -- the destructor doesn't need to be virtual;

 -- inconsistend header guard name.

Change-Id: Ia926ce7601a2166d29d2558ceea51e122ce89232