MethodHandles: Enable return value conversions.

Enables return values conversions and simplify the type conversion
logic. Ensures check for WrongMethodTypeExceptions before invoking a
method handle.

Test: m test-art-host-run-test-956-methodhandles
Bug: 30550796

Change-Id: I8add3ad0a19b43010946b4fb9f1c0f0949287860