Gracefully handle IBluetoothAudioProviderFactory::openProvider failure

openProvider sometimes returns the ok() status but a null
IBluetoothAudioProvider instance. Gracefully handle this case
and treat it as a regular failure.

Bug: 372309179
Bug: 341856622
Test: m com.android.btservices
Flag: EXEMPT, minor bug fix
Change-Id: I90988e36a62336d64396e4fd14700ca48a11b80b