vts_fs_test: checks AVB 1.0 isn't used

AVB 1.0 support in fs_mgr has been removed in commit
Ibfb46aa6c2f761dbb3a9b5f0b16336e510417620.

Adding a VTS test case to ensure the legacy 'verify' fs_mgr
flag isn't used anymore.

Also converting GetFstabPath() to a public API in the fstab.h,
so the test case can read then parse the fstab file.

Bug: 204948957
Test: atest vts_fs_test
Change-Id: Ib6ed7cb8b6ad719b19cd876acf10f4312ecb51b6