Do not use StringPiece in ExtractJavaIdentifier

Do not use StringPiece in ExtractJavaIdentifier

Bug: http://b/91353691

After assigning the result of TransformToFieldName to 'result', the
underlying storage is destroyed after the 'if' statement of the function
call.  'result' ends up with garbage if the identifier has a '-'.

ManifestClassGeneratorTest.NormalizePermissionNames is broken for this
reason in 32-bit Windows when using libc++ and 32-bit Linux.  ASAN also
reports this failure for both 32-bit and 64-bit linux.

Test: Run test on the cases mentioned above and ensure all of them pass.
Change-Id: I69163c423c1171b7ac7838f2abe06bdf8058df4c