Oboe unit tests crashing with memory corruption, library cross linking the flowgraph code

[philburk]

Oboe version: <= 1.4.2
App name used for testing: unit tests

Short description
Unit tests that open and close an MMAP stream crash due to memory corruption.
The root cause is because AAudio is creating an object using the flowgraph code in AAudio,
but calling the destructor in Oboe!
The Oboe and AAudio flowgraph code is similar but not identical. So it crashes.

Steps to reproduce
Run adb logcat in a separate window.
Follow instructions at: https://github.com/google/oboe/tree/master/tests
to run the tests.

Expected behavior
Test runs to completion and and reports number of tests that PASS.

Actual behavior
Test stops before final report.
Logcat shows Native Exception and stack trace like the one below.

Any additional context
See internal b/160633017

I added -fvisibility=hidden and it seemed to fix the problem.

Here is a stack dump:
07-06 14:32:54.969 F libc : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7afffffff0 in tid 11546 (testOboe), pid 11546 (testOboe)

07-06 14:32:55.046 F DEBUG : #00 pc 00000000000444fc /apex/com.android.runtime/lib64/bionic/libc.so (scudo::Allocator<scudo::AndroidConfig, &(scudo_malloc_postinit)>::deallocate(void*, scudo::Chunk::Origin, unsigned long, unsigned long)+132) (BuildId: e0da623f4ab0530a50a9af9e181b341d)
07-06 14:32:55.046 F DEBUG : #1 pc 0000000000074110 /data/user/0/com.google.oboe.tests.unittestrunner/files/testOboe
07-06 14:32:55.046 F DEBUG : #2 pc 0000000000035144 /system/lib64/libaaudio_internal.so (AAudioFlowGraph::~AAudioFlowGraph()+148) (BuildId: 572fc676500947b5f7300082d299c641)
07-06 14:32:55.046 F DEBUG : #3 pc 0000000000033b80 /system/lib64/libaaudio_internal.so (aaudio::AudioStreamInternalPlay::~AudioStreamInternalPlay()+36) (BuildId: 572fc676500947b5f7300082d299c641)
07-06 14:32:55.046 F DEBUG : #4 pc 0000000000003b90 /system/lib64/libaaudio.so (AAudioStream_close.cfi+356) (BuildId: f0582a3365d1efd6d1317e4eb2a35e31)

[philburk]

It is not safe to rely on a build option to prevent a dangerous memory corruption.
We should modify the source code to prevent this.