Resolved incorrect AV startup sequence

Issue:
-> In AV start-up sequence, State machine initialization was being
called after enabling Btif A2dp service.
-> In that case, State machine handle will be empty and it will
prevent enable and register events to enter into BTIF.
-> Because of the absence of enable/register events, AV module in
stack will be unaware of upper layer registration and will not
proceed for connection.

Fix:
-> Changed the sequence in AV start-up, initialized State machine
first before dispatching any events to State machine.
-> So, the handle will not be empty and will not block events like
BTA_AV_ENABLE_EVT and BTA_AV_REGISTER_EVT to enter into BTIF.

Bug: 21451237
Change-Id: I5d47e20ed68ea5730a7c149c316446eb8a1d41af