Cleanup construction of Value instances

Constructors are error-prone due to implicit conversion (char -> int,
...) so we introduce dedicated factory methods instead.

Also allows to remove useless casts (due to safer API).

Test: art/tools/run-jdwp-tests.sh --mode=host --variant=X64
Change-Id: I387eda20a2ff20c06c4a60afcdbfcd4e0392ca28