Add a device config namespace for battery_stats

CpuWakeupStats uses it for new settings. But existing battery_stats
settings should be migrated to it later.

Test: atest FrameworksServicesTests:CpuWakeupStatsTest
Test: Manually inspect the output of `dumpsys batterystats --wakeups`

Bug: 252903682
Change-Id: I011cf8a118fdbaafb29fba9fed349994038585e8