[vold] Add argument verification to IncFS methods

+ Get rid of an extra string copy in path validation function

Bug: 152349257
Test: atest vold_tests
Change-Id: I03a8cab0dd6abd7d5c9dcbbc2acb651e818e6cd8
Merged-In: I03a8cab0dd6abd7d5c9dcbbc2acb651e818e6cd8
6 files changed