Refactor cpu stats pullers

use same cpu stats readers with BatteryStats so that
1) both throttle to avoid too frequent pulls
2) cached value is served within throttle interval to avoid double
pulling by both statsd and BatteryStats

To run unit tests:
bit FrameworksCoreTests:com.android.internal.os.KernelUidCpuFreqTimeReaderTest
bit FrameworksCoreTests:com.android.internal.os.KernelUidCpuClusterTimeReaderTest
bit FrameworksCoreTests:com.android.internal.os.KernelUidCpuActiveTimeReaderTest

make -j56 statsd_test && adb sync data && adb shell /data/nativetest64/statsd_test/statsd_test

Test: cts test, unit test
Fix: 73745189
Fix: 73780619
Fix: 73360959

Change-Id: I10a9bc91ca67fa812f4cd71c4fbd73c1a5ba580e