btm_acl: Only allow automatic flush if packet boundary is supported

Bug: 372035961
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: bluetooth_AdapterAUHealth.au_hfp_nbs_dut_as_source_test.floss
Flag: EXEMPT, behind Floss-only prop (bluetooth.core.classic.auto_flush_timeout)
Change-Id: I0c68e1a3042282b2a3188f2b9b9506f072b643b1