Make KernelSingleProcessCpuThreadReader aggregate CPU times

This is done to
- prepare to switch to a native implementation
- reduce memory allocations

Test: atest FrameworksCoreTests:com.android.internal.os.KernelSingleProcessCpuThreadReaderTest
Test: atest FrameworksCoreTests:com.android.internal.os.SystemServerCpuThreadReaderTest
Test: atest FrameworksCoreTests:com.android.internal.os.BatteryStatsBinderCallStatsTest
Test: atest FrameworksCoreTests:com.android.internal.os.SystemServicePowerCalculatorTest

Bug: 169279846

Change-Id: I4ff41ad7110120836518d86a8c74cd18631e96c7