SECURITY: Reduce precision on os.clock() to mitigate timing attacks

While there is currently no demonstrated vulnerability, this provides
additional hardening against SPECTRE-like attacks, and any potential
future timing attacks.

Bug: T184156
Change-Id: I2b5cc177bded1a9b5600d77116e67817841204be
(cherry picked from commit 6c69cd2ddf4d8bc67b0dae33faaeaeed509d9a3a)