Fix List Properties for Java

This is to imitate current usages and to provide more convenient APIs
for List Properties. In summary,

1) Optional.Empty will return if prop is empty
2) null is permitted as an element of list
3) Elements can be read directly with index

Bug: 80125326
Test: mma -j && sysprop_test
Change-Id: I045a0837b2e38b4107d2236787af4b10b07f7e76