adb-remount-test.sh: Fix failure due to missing host tool `hexdump`

As we are just using it to generate gibberish, we might as well just
hardcode the gibberish in the script.

Also fix unhandled `grep` failure.

Bug: 179752308
Test: Forrest
Change-Id: I534c7cacdb12a104f26d380fe3a571332091490e