android: simplify/improve the attachment of a thread to the Java VM

There is now only one way to get a JNIEnv: jni_get_env. This function use the
thread-specific data (TSD) areas to save a JNIEnv into a TSD key. The thread
will be automatically detached to the Java VM when it's canceled or exited.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>