libpdx_uds: Fix RPC channel ID allocation to not recycle values as often

The value of channel ID for PDX service on UDS transport was actual file
descriptor value for the data socket. Since channels are open and closed
constantly, it is quite often that channel ID is being reused immediately,
so it is almost impossible to use `cid` as unique identifier for objects
being passed around.

Instead, we now use monotonically growing channel ID value.

To prevent the possibility of using channel ID as the socket FD and vice
versa, changed all helper functions that used to take socket_fd as an int
to explicitly use BorrowedHandle which disables implicit conversion from
int (and hence makes it impossible to mistakenly pass in the channel ID).

Bug: 37082296
Test: `m -j32` succeeds for sailfish-eng
      Ran libpdx_uds_tests on the device -> all pass
      Device boots and CubeSea and VrHome render correctly with vr_flinger

Change-Id: Ibb8dfee4d6c3f4b6120c0b6e20a253f1b9307c19