adb-remount-test: Replace libc.so test with build.prop test

Since /bionic mountpoint is deprecated, we don't _have_ to explicitly
check consistency of /system/lib/bootstrap/libc.so anymore.

Remove the test which adds junk to the end of libc.so.  Editing libc.so
looks dangerous (albeit not!) and has unclear expectations.
Add test which edits /system/build.prop. Editing build.prop file is more
"safe" as it's just text edit, and the expectations are clear (edited
system properties should be loaded after reboot.)

Bug: 243116800
Test: adb-remount-test
Change-Id: I51bd32c6ffcc57eb646eeec0537e996847e6c2a5