wifi: Send channel change event to framework

Added a new StaIfaceCallback method to indicate that
the operating frequency of the BSS is changed
due to channel switch announcement from the AP.

Bug: 195617184
Test: Manually triggered channel switch through peer AP hostapd command
      and verified the new frequency in STA logs.
Change-Id: I9dd59d7c5d4ddc5f3443f8b6786cfea9beec4f3c