[runtime] Use statfs(2) on Android.

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=23077

Reverts commit 53ce92470.

Commit 53ce92470 disabled use of statfs(2) on Android, for unspecified
reasons. (My *suspicion* is that the Android version mono was
originally ported to didn't have statfs(2), but I can neither confirm
nor deny that guess.)

Android has supported statfs(2) since API-4 Android v1.6 (in that
statfs(2) is in the header files); what's missing is MNT_RDONLY. What
IS present is MS_RDONLY, which appears to serve the same purpose.

Re-enable use of statfs(2) and use MS_RDONLY when building against the
Android NDK.