[dumpstate] fix -Wreturn-stack-address

ZippedBugreportGenerationTest#getZipFilePath() creates a temporary
std::string, then returns a pointer to the C str within the stack
temporary. Fix by returning a std::string and calling .c_str() at the
call sites of getZipFilePath().

Bug: 139945549
Test: mm
Change-Id: Ie2fc423f74e8ed2cf3f427c4afc3c340ff6c57cc
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>