fs_mgr: adb-remount-test in non overlayfs world

Extend test script to check non overlayfs case too.  Establish
whether overlayfs is supported, or needed, and adjust expectations.
For the flash vendor code, make sure it is the _right_ vendor,
skip instead of fail.

Improve quality of the testing and handle more cases.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Ic9957cb378d8e7737ffc4ba04078c1cb6bbd1bbd