Modify the EXTRA_ACCESSORY_HANDSHAKE_END description

More description of "EXTRA_ACCESSORY_HANDSHAKE_END" to let
people understand.

Bug: 181764810
Test: Build and Boot to home

Signed-off-by: Ricky Niu <rickyniu@google.com>
Change-Id: Ic4cc2bd53bb3c5613893fdf1a4c4e5ed89365144