Fix AVDTP reconfig not setting L2CAP flushable

We set the L2CAP for AVDTP to flushable when opening AVDTP at the first
time, but if the reconfiguration happens, the AVDTP could be run on a
different L2CAP channel which needs to set flushable again.

Bug: 361714491
Test: mma -j
Flag: com::android::bluetooth::flags::fix_avdt_rconfig_not_setting_l2cap
Change-Id: If4e791045a3f9f3010e8402137170dec589e0582