remount: Merge 'remount' and 'set-verity-state'

Dedup fs_mgr_remount.cpp and set-verity-state.cpp by merging them
together and make '/system/bin/set-verity-state' a symlink to
'/system/bin/remount'.

Bug: 241688845
Test: adb-remount-test
Change-Id: I42a2344b9bdac112bf8767d428a99cb020267546