Switch "UTC time" to "Unix epoch time" (core)

This is more correct. Android devices, even under ideal conditions,
don't track UTC (which contains leap seconds and uses the SI definition
of a second) but use a clock based on Unix epoch time (no leap seconds,
but clocks may skip/stop or smear around leap seconds to ensure a fixed
number of second increments per calendar day when leap seconds are
applied to UTC).

Bug: 218802673
Test: Compile only
Change-Id: I0c557e2231f87ea1b99e3935f79af15bccf91da4

core/java/android/provider/Settings.java

@@ -11707,8 +11707,8 @@ public final class Settings {
                 "night_display_forced_auto_mode_available";
 
         /**
-         * If UTC time between two NITZ signals is greater than this value then the second signal
-         * cannot be ignored.
+         * If Unix epoch time between two NITZ signals is greater than this value then the second
+         * signal cannot be ignored.
          *
          * <p>This value is in milliseconds. It is used for telephony-based time and time zone
          * detection.

core/java/android/util/TimeUtils.java

@@ -153,8 +153,8 @@ public class TimeUtils {
      *
      * <p>Time zone database updates should be expected to occur periodically due to
      * political and legal changes that cannot be anticipated in advance.  Therefore,
-     * when computing the UTC time for a future event, applications should be aware that
-     * the results may differ following a time zone database update.  This method allows
+     * when computing the time for a future event, applications should be aware that the
+     * results may differ following a time zone database update.  This method allows
      * applications to detect that a database change has occurred, and to recalculate any
      * cached times accordingly.
      *