Streamline l2c_link_send_to_lower/br_edr and ble

stack/l2cap/l2c_link::l2c_link_send_to_lower

Towards proper interfaces

Bug: 163134718
Tag: #refactor
Test: compile & verify basic functions working

Change-Id: I075af46d4f194fe44c5389b1ab5aa8341a3356ef