adb-remount-test: Refactor fastboot flash vendor test

Instead of relying on a local dev tree (which CI machines never have),
just pull the vendor partition image from device.
This way we can have CI coverage on fastbootd as well.

Stop redefining cleanup() hook, just toss all temporary files to $TMPDIR
and always clean up $TMPDIR on exit.

Clarify logs and error messages.

Bug: 243116800
Test: adb-remount-test
Change-Id: I08fb8df58a61c03db3274b22b51e40a1a8f41095