[SafetyCenter] Update javadocs, add refresh reason constants and update

parcelable implementation

Also address API feedback to change some StringDef to IntDef,
I don't think we need them to be strings, we just need a enumeration.

Test: atest CtsSafetyCenterTestCases
Bug: 222683944
Bug: 210805082
Bug: 221437249
Bug: 219195248
Bug: 219609179
Change-Id: I429cbd975e617afb40cfea32e93b06aac3d66958
Merged-In: I429cbd975e617afb40cfea32e93b06aac3d66958