init: skip early mount in recovery mode

We don't need early mount in recovery mode for security considerations,
e.g., users should explicitly select 'mount /system' from the recovery
menu. This CL checks the existence of file "/sbin/recovery" and skip
early mount when it is found.

Bug: 35853576
Test: early mount /vendor without dm-verity on sailfish
Test: early mount not happen in recovery mode on sailfish
Change-Id: I69cc96f6fd0de6ce493082921738a958dd571115