leaudio: Fix speed configuration when switching LeAudio headset

PendingConfiguration flag was missing to be set when speed configuration
is activated which case no state machine notification on
ConfiguredState, which later blocked AudioHal to resume stream

Bug: 393119197
Flag: Exempt, dat-to-day fix, unittested
Test: atest bluetooth_le_audio_client_test
Change-Id: I37fbb5dfd82b0ac396db02f60648f07614fe9af1