adb-remount-test: Refactor prologue & /sys/module/overlay mining

While doing precondition check, verify that device is debuggable and
unlocked.

The /sys/module/overlay mining code had some remarkably written chained
... && ... || ... expressions. This is also remarkably unreadable for
those untrained of bash command chaining pitfalls.
Just rewrite these with plain old if-then-else expressions.

Bug: 243116800
Test: adb-remount-test
Change-Id: I56b1dea5b9147755a43462682a51bc5802ee64c1