BluetoothAudioHAL: Don't cache the providers factory locally

This object is provided by Audio HAL, and we should get the
service every time while fetching a provider. It fixes the crash
which is due to out-of-sync in case of Audio HAL restarting.

Fixes: 173538746
Tag: #stability
Test: atest bluetooth-test-audio-hal-interface
      restart Audio HAL when Bluetooth is ON
Change-Id: I1e88136c768a1d5fdcd70cfd9e5f1a3a13645333