adb-remount-test: Miscellaneous fixes

* When guessing the ANDROID_SERIAL, use output of `adb devices` instead
  of ro.serialno, because ro.serialno won't work for network devices.
* Ensure ANDROID_SERIAL is exported so the test don't fail if a new
  device is plugged into the host machine mid test.
* Change --wait-screen warning to info. The "warning" isn't helpful as
  it's not showing any potential problems.
* Register cleanup hooks to EXIT trap. This ensures cleanup code are
  always executed, and failure to clean up counts as test failure.
* Rewrite some unnecessarily complex command chaining to plain exit
  status check.
* Use `test` command to test file existence. Don't use `ls` or `cat` to
  test file as this isn't their intended usage, and parsing their error
  output can be finicky.

Bug: 243116800
Fixes: 178256393
Test: adb-remount-test
Change-Id: Iec4224d8a236a9852ce417b1129c27205d435d5b