metricsd: Collect average cpu usage information.

Collect a new histogram (Platform.CpuUsage.Percent), collecting the
average cpu usage in percent of the total cpu usage possible (across all
cpus).
This measurement is collected every minute.

Bug: 25231576
Test: unit tests.
Test: Manual testing.

Change-Id: I1c63486c177f0c2d4bd361eb9e351a7ca25b80d9
diff --git a/metricsd/Android.mk b/metricsd/Android.mk
index b08c153..291c983 100644
--- a/metricsd/Android.mk
+++ b/metricsd/Android.mk
@@ -27,6 +27,7 @@
 
 metrics_daemon_common := \
   collectors/averaged_statistics_collector.cc \
+  collectors/cpu_usage_collector.cc \
   collectors/disk_usage_collector.cc \
   metrics_daemon.cc \
   persistent_integer.cc \
@@ -41,6 +42,7 @@
 
 metrics_tests_sources := \
   collectors/averaged_statistics_collector_test.cc \
+  collectors/cpu_usage_collector_test.cc \
   metrics_daemon_test.cc \
   metrics_library_test.cc \
   persistent_integer_test.cc \