LayerStats: Fix a use-after-free

std::string::c_str returns memory that's freed when the string dies. So,
this method should be returning the string, rather than a pointer to
it's freshly-deleted contents.

Caught by our static analyzer.

Bug: None
Test: Builds

Change-Id: I3dce7b60c6cfb4e3bfbd6c0dd6fe30dfe7f4313c