Make IPowerStats nullable in SystemHealthManager

"powerstats" service is unavailable in the ephemeral mode,
therefore it needs to be allowed to be missing.

Test: atest --instant CtsOsTestCases:android.os.health.cts.SystemHealthManagerTest
Bug: 288124045

Change-Id: If6bfe6a2e445670f01b16893c1dc31269bbcb30b