Conservatively trim data usage stats.

Instead of trusting NTP time alone, use the most-conservative of
system clock and NTP.

Bug: 5584564
Change-Id: I5dd87fc009959b1cf0a7d660e385a0b1a8be238b
diff --git a/core/tests/coretests/src/android/net/NetworkStatsHistoryTest.java b/core/tests/coretests/src/android/net/NetworkStatsHistoryTest.java
index e1db073..1df763a 100644
--- a/core/tests/coretests/src/android/net/NetworkStatsHistoryTest.java
+++ b/core/tests/coretests/src/android/net/NetworkStatsHistoryTest.java
@@ -256,6 +256,10 @@
         stats.recordData(TEST_START, TEST_START + DAY_IN_MILLIS, 24L, 24L);
         assertEquals(24, stats.size());
 
+        // try removing invalid data; should be no change
+        stats.removeBucketsBefore(0 - DAY_IN_MILLIS);
+        assertEquals(24, stats.size());
+
         // try removing far before buckets; should be no change
         stats.removeBucketsBefore(TEST_START - YEAR_IN_MILLIS);
         assertEquals(24, stats.size());