Do not use AAudio unless running on Android 8.1 or higher

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.

src/aaudio/AudioStreamAAudio.cpp

@@ -16,12 +16,16 @@
 
 #include <assert.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 #include "aaudio/AAudioLoader.h"
 #include "aaudio/AudioStreamAAudio.h"
 #include "common/OboeDebug.h"
 #include "oboe/Utilities.h"
 
+#ifdef __ANDROID__
+#include <sys/system_properties.h>
+#endif
 
 
 using namespace oboe;
@@ -89,7 +93,22 @@ AudioStreamAAudio::~AudioStreamAAudio()
     delete[] mShortCallbackBuffer;
 }
 
+static bool isRunningOnAndroid8_1OrHigher() {
+#ifdef __ANDROID__
+  char sdk[PROP_VALUE_MAX] = {0};
+  if (__system_property_get("ro.build.version.sdk", sdk) != 0) {
+      return atoi(sdk) >= 27;  // SDK Level 27 is Android Oreo 8.1
+  }
+#endif
+  return false;
+}
+
 bool AudioStreamAAudio::isSupported() {
+    if (!isRunningOnAndroid8_1OrHigher()) {
+        // See https://github.com/google/oboe/issues/40,
+        // AAudio is not stable enough on Android 8.0.
+        return false;
+    }
     mLibLoader = AAudioLoader::getInstance();
     int openResult = mLibLoader->open();
     return openResult == 0;
@@ -380,4 +399,4 @@ Result AudioStreamAAudio::getTimestamp(clockid_t clockId,
     }
 }
 
-} // namespace oboe
+} // namespace oboe