adb: Preserve the original mount flags when remounting

This change preserves any additional flags mount flags (e.g. MS_NODEV)
that any of the mounts would have. This avoids unnecessarily opening up
permissions, and also allows kernels that have additional restrictions
about what mount flags can be used to be happy with the remounts.

Bug: 111618714
Test: `adb remount` works in Chrome OS
Test: `adb remount` works in sailfish_aosp

Change-Id: I20d9f2feaf3a47b93bfcdfb4164ee61546ec0b68