Android: Suppress -Wattributes warnings in test case builds

We use `-Werror` in the Android test builds to make sure there are
no warnings that we care about (e.g. unused flags).  However, the
NDK r13 tools produce a warning about their own builtins:

```
<built-in>: In function 'float abs(float)':
<built-in>: warning: conflicts with previous declaration here [-Wattributes]
```

Suppress this warning so that we can continue using `-Werror` but
tolerate these warnings.

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