Android: change the way JNIEnv is retrieved

- Currently, it tries to Attach the JVM first and update the thread local storage, which are two operations.
  Now, it simply gives back the JNI Env stored for the thread.

- Android_JNI_SetupThreadi() should only be used for external.
  For internal SDL thread, it's already called in RunThread() (SDL_systhread.c),
  and other thread are Java threads which don't need to be attached. i
  (even if it doesn't hurt to do it, since it's a no-op).

- JNI_OnLoad is filled with pthread_create, GetEnv, AttachCurrentThread...
  It's called for all shared libraries which may don't want this setup,
  and loading libraries can be also modified to be done from a static context,
  or with relinker. So it's not really clear how, who and what it sets up.
  => Reduce this function to the minimal
1 file changed