adb-remount-test: check vendor image signature before using

When developing and using the adb remount test, if device under test
is flashed from another source than the current visible tree, make
sure that the vendor image as-built and visible in a sandbox build
is not used indiscriminantly.

Test: adb-remount-test.sh
Bug: 132070014
Change-Id: I30569a7c871f4c4038b0f7f9c05f5f1a5d12c766