AutoOn: Clear stored timer when setting is toggled

When toggling the UI toggle button, we want the value of the timer to be
recomputed and take into account only the last activation

Bug: 323060869
Flag: com.android.bluetooth.flags.auto_on_feature
Test: atest ServiceBluetoothRoboTest
Change-Id: Ibd740f0b97eeeedbe087dc7b56c1b5ffd37c5cd1