libudx: Move ServiceDispatcher from libpdx_uds to libpdx

The two separate implementations of ServiceDispatcher for UDS and ServiceFS
are practically identical. Moved the implementation into the base libpdx lib,
and removed the transport-specific versions.

Bug: None
Test: `m -j32` succeeds; pdx_tests, pdx_servicefs_tests, pdx_uds_tests pass
Change-Id: I2344f4f23bc3da27eb7b192344844b87660e8610