Refer to the last Hotspot password when generating the password

- Query the last Hotspot password when security type is SECURITY_TYPE_OPEN and it will be used when the user changes to another security type

- If the last hotspot password is an empty string, then generate random password

Bug: 201789473
Test: manual test
atest -c WifiHotspotRepositoryTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherPasswordPreferenceControllerTest

Change-Id: Id3a846099238433e7b6c8b934fa26a74df0dcb61