Fix ProtectedProperty to immutable for preserving immutablility on any execution context

[FrancescoJo]

AS-IS

class com.sirloin.jvmlib.util.ProtectedProperty<T>(
    var value: T,
    // ...
)

TO-BE

class com.sirloin.jvmlib.util.ProtectedProperty<T>(
    val value: T,
    // ...
)

MOTIVATION

Even if we declared value as val, immutablility will be broken if the target is mutable. Assigning final mutable collections or an arbitary object which holds some mutable fields, for example.

The current ProtectedProperty#value declared as var is a meaningless attempt - even worse, it breaks the immutability of the client code.

Therefore, change value to val so that mutability management can be left to the client code.

IMPACT

ProtectedProperty value changing codes will not compile after this fix.