Doc that SharedPreferences is IO-expensive

Tried to find the right middle ground in between
- not exposing potentially changing implementation details
- being clear on what it is for and what not
- being too alarmist vs. being to weak

Test: compiled
Bug: 69177691
Change-Id: Ifb40e350a18bf72c113229f31e129b7e8fdacd2a
1 file changed