Revert "Revert "Use IsAssignableFrom instead of IsSubclass for robustness.""

Don't use IsAssignableFrom, but check beforehand if the referrer is an
interface. Otherwise, we are being too aggressive on removing clinit
checks on interfaces (being a subclass doesn't imply the interface
has been initialized).

bug:21870666

This reverts commit 463580ca.

Change-Id: I2a13974d28477ee3e5224142cc260cbf46d19aa9