Align the time of bssid comparison to avoid NPE caused by value changes.

This CL contributes the follwing refinements:

- The return value of `wifiInfo.getBSSID()` may change when calling
outside or inside `synchronized`, so it would be better to migrate the whole `bssid` comparison inside that `synchronized`.

- Use `TextUtils` to handle `bssid`

Bug: 315308336
Test: m
Change-Id: Ie6fce634c9f097e9a866ec44d6de1e5d811a6b3f