Fix failed test CommandContentProviderTest.

Tests failed when running atest
SettingsGoogleRoboTests:com.google.android.settings.fuelgauge because
instance of PowerAllowlistBackend has been initialized by other unit
tests.
Reset it before the unit tests to avoid the issue.

Flag: TEST_ONLY
Test: atest SettingsGoogleRoboTests:com.google.android.settings.fuelgauge
Bug: 349693853
Change-Id: Ia9dd4739665fa1ac0d48ba36a9417d249157cde6