[le audio] BassClient check and reset mPendingMetadata

mPendingMetadata is used to track the source add/update gatt operations.
It should be reset properly with the gatt response. And after being set to the current metadata, it should also be reset properly. Otherwise, it
might cause the hasPendingSourceOperation return unexpected state.

Bug: 316423019
Bug: 316005152
Test: atest BassClientServiceTest BassClientStateMachineTest
Test: manual test with le audio broadcast QR code path
Change-Id: I139f792c64fd848ade81f1d852c6e9e09da06681