Do not use AAudio unless running on Android 8.1 or higher

[mnaganov]

From #40, it seems that AAudio
on Android 8.0 isn't stable enough. As a safety measure, always
fall back to OpenSL ES on it.

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[mnaganov]

Please note that I actually work for Google, just using my personal GitHub account. I've signed the CLA just in case, but I believe it's not really needed in this case.

[googlebot]

CLAs look good, thanks!

[dturner]

Tested on Pixel (emulator) running API 26. Result: OpenSL ES is used - PASS
Tested on Pixel 2 running API 27. Result: AAudio is used - PASS

+2 Approved