Return std::string to not point to freed memory

Since the code was changed to use std::string instead of String8, it is
no longer safe to pass a raw pointer to internal memory of the string
object, since std::string creates a copy even if the data itself is not
being modified.
Instead, return std::string to the calling function and let the function
access the pointer when necessary.

Bug: 71541263
Test: boot up asan build on pixel 2 xl, then interact with google
calendar and chrome. Observe no crashes with the patch.

Change-Id: I50311085b50eab73e71409f6446213d2ff04bc47