LeAudioService: Make sure background scan uses PHY 1 M

Minor improvement for some platforms which would use also Coded if PHY
is not explicity set, which could increase power consumption.
Idea for this scanner was to run on PHY 1M anyways.

Bug: 289885994
Test: mmm packages/modules/Bluetooth
Flag: Exempt, trivial fix, not functional change as LeAudio devices uses
PHY 1M

Change-Id: Idbc53552a40927d41f530013364c6454ccb6b3c6