ART: Refactor String.init binding

Do not use JNI infrastructure, and move the initialization to
the classlinker's FinishInit. This means the binding is available
after the minimal set of initializations.

We could consider moving the functionality even earlier, as soon
as String methods are known and StringFactory can be loaded, but
this placement works well enough and is nicely isolated.

Bug: 79902155
Test: m test-art-host
Change-Id: If4e9f1424668eb200ff5544bc21f5cea6151e4b3