adb-remount-test: Make awk scripts mawk-v1.3.3-compatible

Our CI is failing because the host machine is using mawk instead of gawk.
mawk v1.3.3 cannot parse regex such as '/[/]/', while mawk v1.3.4 and
gawk can.
Change regex of '[/]' to '\/' so that our test script is as backward
compatible as possible.

Bug: 188862155
Test: Run adb-remount-test.sh on CI
Change-Id: Ia4fbce58a61325a5e5280ede0d5b7760832d8ec1