MediaMetrics: Remove remaining BOOTTIME references

Use SYSTEM_TIME_REALTIME instead.
As we always specify the time when the item arrives,
this has no real effect, just for internal consistency.

Test: atest mediametrics_tests
Bug: 149850236
Change-Id: I34d7bed0c4e25b288493ce1ecf3574c4d87334d9
diff --git a/services/mediametrics/MediaMetricsService.cpp b/services/mediametrics/MediaMetricsService.cpp
index d27fa5e..a6fefd2 100644
--- a/services/mediametrics/MediaMetricsService.cpp
+++ b/services/mediametrics/MediaMetricsService.cpp
@@ -386,7 +386,7 @@
                 break;
             }
             if (now > when && (now - when) <= mMaxRecordAgeNs) {
-                break;  // TODO: if we use BOOTTIME, should be monotonic.
+                break; // Note SYSTEM_TIME_REALTIME may not be monotonic.
             }
             if (i >= mMaxRecordsExpiredAtOnce) {
                 // this represents "one too many"; tell caller there are