Add PH flag for P power model

Add P/H experiment flags for Android P power model. The flags will share
the existing Settings.Global.BATTERY_STATS_CONSTANTS. Key
"track_cpu_active_cluster_time" controls KernelUidCpuActiveTimeReader
and KernelUidCpuClusterTimeReader. Key "read_binary_cpu_time" will be
used in a future optimization.
Bug: 68982314
Test: local build and manual test

Change-Id: Ie84fd5265dc65dd0d46ac347cefa32ad92d643ee