update_engine: Use clock and fake clock from SystemState

No need to pass clock and fake clock anywhere anymore. This CL makes it
to just use those objects available from SystemState and
FakeSystemState.

BUG=b:171829801
TEST=cros_workon_make --board reef --test update_engine

Change-Id: I9a3cf6dd2057620c11b862d3317b83489c76f3ca
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2546625
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/common/clock.cc b/common/clock.cc
index 05c495c..0821a56 100644
--- a/common/clock.cc
+++ b/common/clock.cc
@@ -20,11 +20,11 @@
 
 namespace chromeos_update_engine {
 
-base::Time Clock::GetWallclockTime() {
+base::Time Clock::GetWallclockTime() const {
   return base::Time::Now();
 }
 
-base::Time Clock::GetMonotonicTime() {
+base::Time Clock::GetMonotonicTime() const {
   struct timespec now_ts;
   if (clock_gettime(CLOCK_MONOTONIC_RAW, &now_ts) != 0) {
     // Avoid logging this as an error as call-sites may call this very
@@ -40,7 +40,7 @@
   return base::Time::FromTimeVal(now_tv);
 }
 
-base::Time Clock::GetBootTime() {
+base::Time Clock::GetBootTime() const {
   struct timespec now_ts;
   if (clock_gettime(CLOCK_BOOTTIME, &now_ts) != 0) {
     // Avoid logging this as an error as call-sites may call this very