Only open a path that is a regular file

... and return a fixed errno before the file path is authenticated.

To avoid a malicious client blocking installd with a path to FIFO,
check the file type before opening it. A corresponding test isn't added
because it's hard to test that the FIFO has never been opened by path.

Bug: 320682896
Bug: 319284987
Test: atest FsVerityTest installd_service_test
Change-Id: Icd30cca809a45447444842ec277b2ac536f26d88