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
Bug: 73745189
Bug: 73780619
Bug: 73360959

Merged-In: I10a9bc91ca67fa812f4cd71c4fbd73c1a5ba580e

Change-Id: I10a9bc91ca67fa812f4cd71c4fbd73c1a5ba580e