ObjPtr<>-ify String allocations, fix stale refs.

ObjPtr<>-ify String allocation functions and related code
and remove some unnecessary calls to ObjPtr<>::Ptr(). Fix
stale reference uses in reference_table_test and stub_test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: I42927fb8b7240e5132188f73318b2ccb218748fd
18 files changed