Add ParseFstabFromString(), remove ReadFstabFromFp()

ReadFstabFromFp() have two callers right now, ReadFstabFromFile() and
ReadFstabFromDt(). ReadFstabFromFile() opens a FILE* and pass it to
ReadFstabFromFp(), and ReadFstabFromDt() wraps a std::string::c_str()
buffer in a FILE* adaptor with fmemopen().

There's no need for such adaptor, just change ReadFstabFromFp() to
accept std::string and we're good.

Bug: 204056804
Test: atest CtsFsMgrTestCases
Test: m libfstab_fuzzer
Change-Id: I71190c7356a354aa06d1be3fdc4ca76611b3896d