adb-remount-test: Refactor remount RW test

* Check mount flag changes (ro/rw) before and after "adb remount".
* Add comment explaining what's going on with the |uses_dynamic_scratch|
  and |scratch_partition| variables.
* Add rich logs reporting infomation about the scratch partition.
* Add rich error messages.
* Filter out /data devices and external volumes (vold managed device)
  when checking RW partitions. We are only interested in system
  partitions.
* Remove redundant "remount from setup" test from end-of-file, as they
  are testing the same thing as the refactored remount test. Total test
  runtime reduced by one reboot.

Bug: 243116800
Test: adb-remount-test
Change-Id: Icda5bff78372bebfe2e166d8537a06be66fff886