Also show "last BS off time" in dumpsys power

Change-Id: I53d7727847b84b29bdc8285c50ce59def4378684
Fix: 75024658
Bug: 75024658
Test: Manual test with dumpsys power
Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/BatterySavingStatsTest.java