IssueTracker

The following code, when ran ~100 times will produce between 3-5 assertion failures.

```
public void basicTest() throws Exception {
  if (android.os.Build.VERSION.SDK_INT < android.os.Build.VERSION_CODES.M) {
    throw new RuntimeException("API v23 or higher is required to run this test");
  }
  KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
  keyStore.load(/* param= */ null);

  final ArrayList<Exception> exceptions = new ArrayList<>();

  Thread thread =
      new Thread() {
        @Override
        public void run() {
          try {
            MasterKey masterKey =
                new MasterKey.Builder(ApplicationProvider.getApplicationContext())
                    .setKeyScheme(MasterKey.KeyScheme.AES256_GCM)
                    .build();
          } catch (Exception e) {
            synchronized (exceptions) {
              exceptions.add(e);
            }
          }
        }
      };

  // This starts a new thread that generates a new master key. I think that thread blocks until
  // the new master key is finished.
  thread.start();

  // We now try to create another master key in this thread.
  MasterKey masterKey =
      new MasterKey.Builder(ApplicationProvider.getApplicationContext())
          .setKeyScheme(MasterKey.KeyScheme.AES256_GCM)
          .build();

  // This should be true, but it is sometimes false.
  assertThat(keyStore.containsAlias("_androidx_security_master_key_")).isTrue();

  // Wait for other thread to finish
  thread.join();

  if (exceptions.size() > 0) {
    throw exceptions.get(0);
  }
}
```

This is because of a non-atomic check and set in `MasterKeys.getOrCreate`.

Reported by juerg@google.com