[Catalyst] Reduce memory footprint for SettingsPreferenceValue

Current implementation has following issues:

- Not scale to value types. Each preference has only one value
  (according to its type) but the class has fields for ALL value types,
  which increases the memory usage unnecessarily.
- Use internal API {read/write}String8 for string type.
- Missing type check.

Fix: 389778788
Flag: com.android.settingslib.flags.settings_catalyst
Test: atest CtsSettingsPreferenceServiceTest
Change-Id: I1dd44370bc80f271564ffe60b7c5700dbfc64a15