Use Thread.dispatchUncaughtException() rather than duplicating logic.

Previously, the runtime duplicated the logic in Thread's methods
dispatchUncaughtException() and getUncaughtExceptionHandler().
Since we're modifying this logic to ensure that uncaught
exceptions are logged, this CL changes the runtime to call
dispatchUncaughtException instead. This also has the benefit
that we can remove Thread.UncaughtExceptionHandler from the
list of well known classes.

Test: art/tools/run-libcore-tests.sh --mode=host --variant=X32
Bug: 29624607
Change-Id: Id7161d841d64ff9d8f2a7ec1c8c0c5c911ff0b64