Reduce the number of Send requests over UDS per IPC invocation

We sent up to 3 separate send requests over a socket. Now combine
then into one sendmgs() to improve performance and thread scheduling.

This improves PDX/UDS performance by up to 2x in some cases.

Bug: 65379030
Test: Sailfish works normally. Was able to run VR apps.
Change-Id: I00734d2c3f06d7d42b089569a8cc11c2273a1b7f