Set the AVRCP Browsing MTU for a device after browse open

Trying to get the Browsing MTU before the Browsing channel is opened
returns the minimum MTU. This leads to smaller browsing messages than
expected.

Bug: 113160447
Test: Connect to device and see that Avrcp Browse packets are the
correct size

Change-Id: Iefda855d0a777439a8a6829619bc4c5543b62eaf