os: Fix GetTimeInMicros resolution

GetTimeInMillis is called first, which sets clockid to
CLOCK_MONOTONIC_COARSE, which is typically much lower resolution than
the callers of GetTimeInMicros want.

Prior to a779fda2, GetTimeInMillis and
GetTimeInMicros did not share a clockid.

Restore the clockid split to fix the granularity of GetTimeInMicros.

Signed-off-by: Peter Harris <pharris@opentext.com>
(cherry picked from commit 937a5b78)