Linux:  Revert c47c790f and instead defer modifications to the fdReadCallbacks vector when inside a read callback until after the callback has been executed

On iOS/arm7, double has an alignment of 8 but std::max_align_t has
alignment of 4, so we need to compute our own max_align value.

Android:  Pulled out some shared OpenSL/Oboe code into juce_android_HighPerformanceAudioHelpers.h and updated Oboe buffer size behaviour to match OpenSL