Fix some pages crash after importing AndroidX from build 5175906

- Settings crash while entering Storage, Privacy, and Accounts page,
  because PreferenceGroup changed the API use from String#equals to
  String#contentEquals which doesn't support null keys.
- add a null check before calling findPreference

Test: robotest
Change-Id: I121cd9e4249fbdafbc67be65a09d770603e01044
Fixes: 121116425