libaudiohal@aidl: Allow unspecified offload bit rate

The average bit rate for offload is not always provided
by the framework. In this case the default value of `0`
is used.

Bug: 323313634
Test: atest android.media.audio.cts.AudioTrackOffloadTest
Change-Id: I93708a2718d74749392002b00e12f3ac1f9ae057