Export BatteryUsageStats into prepopulated buider

Instead of overriding what has already been captured in the builder,
always add new estimates. This way we can accumulate BatteryUsageStats
without allocating new CursorWindows.

Bug: 366493365
Test: atest PowerStatsTests
Flag: EXEMPT bugfix
Change-Id: Ica681e20c1091c2336532e58583ca5d3f55db85f