Ensure BatteryUsageStats test isolation in regards to instance leak detection

If one of the unit tests failed, its BatteryUsageStats instance could
leak, only to be detected when the next test method is executed,
wrongly attributing the leak to that other test.

Bug: 403030823
Test: atest PowerStatsTests;atest PowerStatsTestsRavenwood
Flag: EXEMPT test only
Change-Id: Ifa4cb1f29559785f9de1dde709d2c24da070d51a