Remove class root duplicates from well known classes.

And get well known exception classes as declaring classes
of their constructors.

Also change function `ThreadForEnv()` to `Thread::ForEnv()`
and use it where appropriate, mostly in code added recently
while cleaning up well-known methods.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I2ededa429863a6cddbcbb879a223277fd6245557