Use std::string in DeviceProductInfo and serialize it as Flattenable

Use std::string instead of fixed size char arrays. Serialize
DeviceProductInfo and DisplayInfo as Flattenable instead of using
memcpy().

Bug: 145299597
Test: 1. m
      2. adb shell dumpsys display
      3. check that DeviceProductInfo is correctly populated
Change-Id: Id21186138b39d7bb167c41ff7ee9387081ac6285