Replace use of legacyInitialValue() with constantValue

`FieldItem.legacyInitialValue()` is being removed and replaced with
`FieldItem.constantValue`. This change updates the systemfeatures
tool to use the latter.

Note: The checks for `isStatic()` and `isFinal()` were removed as
`constantValue` is only non-null on `static final` fields anyway.

Flag: EXEMPT updating new build tool
Bug: 354633349
Test: ./gradlew
Change-Id: I949de20d59526c2f3b45d7f1c4d9df8d15b2f51c