Use vector instead of raw pointer in l2cap socket write handling

Currently, when we receive data from upper layers for LE CoC socket,
we store it in memory and pass a raw pointer around, keeping memory
until the write is acknowledged by controller and event is sent back to
btif layer in on_l2cap_write*_done. If anything goes wrong, we might
loose the pointer to the data and have a memory leak.

From now on, we store the data in std::vector and pass its ownership
down the stack. The moment the data is copied into lower layer buffer,
vector is automatically freed. We no longer pass the data back to
on_l2cap_write*_done callback.

Bug: 68359837
Test: compilation test
Change-Id: If34c0fba8d4a040a242b8c655491b8967a03b96a