Update AudioFlinger::setBluetoothVariableLatencyEnabled

In a situation when the primary module does not support variable
latency, but some other module does, none of current playback
threads might be able to enable variable latency. It should not
be an error if these theads are from modules that do not support it.

Flag: EXEMPT bugfix
Bug: 413497881
Test: atest --test-filter=".*testBluetoothVariableLatency.*" CtsMediaAudioTestCases
Change-Id: Ia38035ee6cf77634a09562e115f0e65fa34a9d06