Disallow modifying java.lang.String

The optimizations we do for j.l.String mean that if agents put
profiling code j.l.String they will see a lot of surprising behaviors
(for example the String constructor is never called).

Bug: 35396224
Test: ./test/testrunner/testrunner.py --host -j40
Change-Id: I30df68186f30f7311e2a5e36b18d807d8e91a596
3 files changed