BassClient: Allow only one pending source operation

This is required by the API design so that a single device
could have a single pending source operation that is
about to end with an appropriate callback call.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
Test: atest BluetoothInstrumentationTests
Change-Id: I2f2054c6891e171ae04e677b3fe72181d6fb6242