Implemented a new method for walking string references in AppImages.

* Defined a new section in the AppImage binary format.
* Verify heap invariants having to do with native string references.
* Collect managed and native references to managed strings.
* Record the location of these references as offsets into the image.
* Writes offsets into new image section.
* Walk this list of offsets at load time to intern strings from the
AppImage.
* Verify that strings have been interned correctly.

Test: art/test/testrunner/testrunner.py -b --host
Test: m test-art-host-gtest
Test: Built and ran Keep, Docs, Chrome, YouTube, and Facebook.
Bug: 70734839

Change-Id: Ib5235abdebca018e8920d12318c31b9c0efb0ede