Improve BT HAL write response to BT server reads

By polling, we ensure the socket buffer is nearly always full,
as blocking socket writes may not wake in time to avoid underflow.

Bug: 28286313
Change-Id: I056768dbd860993c41d44c33a4846d62c50db933
(cherry picked from commit 5952e40bef1afb310cd3bba7e673163d7e32d62c)