Instead of introducing a global variable, use the already existing
saved android_app pointer in lo-bootstrap.c, and just add a function
to rettrieve that.

Reanme osl/detail/android.h back to android_native_app_glue.h, which
is the file from NDK/sources that it is. "android.h" sounded to me too
grand, as if it was some universal Android header. But if we do start
to modify the android_native_app_glue stuff heavily, then it indeed
makes sense to call it something else. Until then, revert also some
whitespace changes to android_native_app_glue.c for it to be as close
as possible to the "upstream" one in the NDK, for clarity.