method_handles: Minor refactor of PerformArgumentConversions.

Separate out a method that can perform a single argument conversion.
Useful for Field setters and also for future use in transforming handles
where the input source will not be a stack frame.

Also remove unnecessary JValue* argument.

bug: 30550796
Test: make test-art-host
Change-Id: I75a63800839dbf1016a2c362169e138c83f34e4c