Try locking after mounting metadata

In rescue mode, if /metadata is mounted but /metadata/ota does
not exist, immediately unmount /metadata and fallback to the code
path when /metadata is not mounted; that is, old partitions are
overwritten.

Test: in recovery, select wipe then immediately sideload
Bug: 160457903
Change-Id: I412d62b7005c81a7126106edc471622e6a7ef813