Dependency on NDK r16 is probably too strict

[dturner]

Run some tests to verify the earliest version of the NDK which can be supported.

[mnaganov]

You definitely need AAudio header files which appeared in r15. I used r15c for the Loopback app with AAudio support.

[dturner]

There are also some other dependencies on macros and methods which only appeared in r15. I will update the docs to say Oboe will work with NDK r15 but not r14 and below.

[dturner]

Docs updated in commit 7621b5d