MediaExtractor: Fix use-after-free

ZipString only references the given string and does not copy it.
It is thus important that the caller keeps the source, in this
case the formatted String8, alive.

Bug: 72329184
Test: m
Test: m && m SANITIZE_TARGET=address ; no aborts
Change-Id: I20f9acd544eeeb26cc115b753ac64b3bce6235b6