JUCE is determining default device incorrectly

For context, see https://github.com/google/oboe/issues/707#issuecomment-558991551

JUCE determines the default device during initialization of juce::AudioDeviceManager. But the default can change if the user plugs in or out a headset. They should use kUnspecified for the default device.

[juce_android_Oboe.cpp line 1074 getDefaultDeviceIndex()](https://github.com/WeAreROLI/JUCE/blob/35023af7ffe7e9aeaa43538d14d3d199d0ff2f3f/modules/juce_audio_devices/native/juce_android_Oboe.cpp#L1074)