adb-remount-test: Redirect all test output to stderr

Right now some test output are print to stdout and some to stderr.
Stdout mostly contain output of test commands.
Stderr mostly contain test result and device diagnostic status.

The logs in both streams also don't have timestamps, so separating the
two streams would be incredibly unuseful, because it would be very
difficult to deduce the causuality between the log lines.

In practice only the concatenated log stream is useful, so let's just
redirect all meaningful logs to stderr for good measure.
Why not stdout? Because stdout is often captured by command
substitution as command output.

  foo() {
    echo "Log nessage..." >&2
    echo "function output..."
  }
  A=$(foo)

"Log message..." would go to stderr, and "function output..." would be
captured into ${A}.

Bug: 243116800
Test: adb-remount-test
Change-Id: I692a1b6cf352681cca65354688908e4becf9d31a