Remove dependency on libutils in native autogen

Right now, Q schema generated code has a dependency on libutils.
Turning on R schema for clients will remove libutils as a required
dependency. Once libutils is removed from clients, turning the flag off
again will be complicated as all clients will need to re-include
libutils. To fix this problem, remove dependency on libutils in Q
schema.

Right now, we only use SystemClock.h from libutils in Q schema autogen.
Use timespec from C standard library instead.

Bug: 146808265
Test: m
Test: flashes successfully
Test: adb logcat "*:S statsd:*"
Test: bit statsd_test:*
Change-Id: I00efeab1be351a95083f3b6e2dbc9067849bee20