Revert^2 "Improve mirror::String implementation."

This reverts commit 15efe16175115f29e7c99bfc2703c30a82e3f702.

Fixed copy-paste error in AllocFromStrings, added tests
for String.concat() to 021-strings2 as regression test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 169674485
Bug: 171273669
Change-Id: I4b298020b7b31de84898243ad73db40a8ad7d69e
2 files changed