Android: OS.create_instance() should return -1 on failure

[dsnopek]

I was checking if OS.create_instance() worked in exported Android games (it does not - it's unimplemented) and found that it returns 0, however, according to the docs it should return -1 on failure.

Also, within the engine itself, the OS::get_singleton()->create_instance() returns Error rather than the PID integer, and that would return OK in exported Android games, whereas it should probably return FAILED, because it doesn't actually work.

A super minor thing because it's unimplemented, but if someone wrote cross-platform code and expected -1 in the case of failure, it could potentially create problems.