Replace CLOCK_MONOTONIC with SYSTEM_TIME_MONOTONIC

Using SYSTEM_TIME_MONOTONIC works for Android (where it translates to
CLOCK_MONOTONIC) and host targets, while CLOCK_MONOTONIC is not defined
on macOS.

Bug: 117921091
Test: existing tests should pass
Change-Id: I1fad472881830fb0701a320cf37319e083932ad4