Upgrade expat to R_2_6_2

This project was upgraded with external_updater.
Usage: tools/external_updater/updater.sh update external/<absolute path to project>
For more info, check https://cs.android.com/android/platform/superproject/+/main:tools/external_updater/README.md

Test: TreeHugger
Change-Id: Iae1249f1796580de4bb3332b53d7cebcb7b4a45b