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
diff --git a/libs/hwui/renderthread/RenderProxy.cpp b/libs/hwui/renderthread/RenderProxy.cpp
index 1a1b9da..20247dc 100644
--- a/libs/hwui/renderthread/RenderProxy.cpp
+++ b/libs/hwui/renderthread/RenderProxy.cpp
@@ -339,7 +339,7 @@
     };
     nsecs_t lastVsync = renderThread->timeLord().latestVsync();
     nsecs_t estimatedNextVsync = lastVsync + renderThread->timeLord().frameIntervalNanos();
-    nsecs_t timeToNextVsync = estimatedNextVsync - systemTime(CLOCK_MONOTONIC);
+    nsecs_t timeToNextVsync = estimatedNextVsync - systemTime(SYSTEM_TIME_MONOTONIC);
     // We expect the UI thread to take 4ms and for RT to be active from VSYNC+4ms to
     // VSYNC+12ms or so, so aim for the gap during which RT is expected to
     // be idle