stack/a2dp_ext: Update selectable capabilities in A2dpCodecConfigExt#setPeerCodecCapabilities

setPeerCodecCapabilities is invoked when the peer codec capabilities
are received. Also the configuration cannot be determined without
the hal audio provider, the selectable capabilities need to be updated
to reflect the fact that this codec is supported by the peer

Bug: 396235402
Test: m com.android.bt
Test: QC validation
Flag: EXEMPT, minor bugfix
Change-Id: Ic1900ef33a87ebe7b88e49108f3e29be8e6bd6c4