Track per-cluster times of each uid in microsec.

Bug: 62805443
Test: Used some apps and verified that battery usage is attributed to
      these apps in Settings>Battery.

Change-Id: I0e604fdc0d1704d7d061b6a711a0017b82a39b8f
diff --git a/core/java/android/os/BatteryStats.java b/core/java/android/os/BatteryStats.java
index 54d5860..7d664ec 100644
--- a/core/java/android/os/BatteryStats.java
+++ b/core/java/android/os/BatteryStats.java
@@ -686,7 +686,7 @@
         public abstract long getSystemCpuTimeUs(int which);
 
         /**
-         * Returns the approximate cpu time (in milliseconds) spent at a certain CPU speed for a
+         * Returns the approximate cpu time (in microseconds) spent at a certain CPU speed for a
          * given CPU cluster.
          * @param cluster the index of the CPU cluster.
          * @param step the index of the CPU speed. This is not the actual speed of the CPU.