Improve mirror::String implementation.

Explicitly use string compression to optimize uses of
String::CharAt() in string.cc similarly to
    https://android-review.googlesource.com/1446657 .
Move string content initialization to pre-fence visitor.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 169674485
Change-Id: I3dc453ce995d09f996610a1eba5060bacbe0d498
1 file changed