TransformFstabForDsu() skips handling special devices

Don't try to find a scratch partition in the fstab as it is a special
devices initialized by fs_mgr_overlayfs during "adb remount".
Don't hardcode the string "userdata_gsi", instead use the constant
kDsuUserdata defined in libgsi.h.

Bug: 165925766
Test: Build pass.
Test: Install and boot a DSU.
Change-Id: I4eaff9461e87e49704c976ddf2a096642957cf56