Effect config parser: fix use after free on file path

ParsingResult::configPath is the path of the configuration file
used for the factory config parsing.
This path is used for an error log if the configuration file has errors.

The paths used to be a static string literals stored as char*
without lifecycle management.
When it was changed to dynamic strings, the code was not updated.

This patch changes it to a std::string.

Bug: 111261328
Test: flash and check effect works
Change-Id: Ia2022c794936f3f75793371cdde86c3047bb6c0a
Signed-off-by: Kevin Rocard <krocard@google.com>