Fix compile error & clean-up codes

There has been compile errors of generated cpp files if we use writable
EnumList prop, as element formatter should be prior to list formatter,
but for now FormatValue(vector<T>) is defined before FormatValue(enum).
They are reordered so that generated cpp files compile well.
In addition, removed some unnecessary codes / and refined codes for
String property to not make redundant copies.

Bug: 80125326
Test: mma -j && run sysprop_test
Change-Id: I1cb4007e8146aff8db90779fd506e364f9e02f4a