Revert "Remove unused method downgradeDatabase"

Revert "[automerge] Remove unused method downgradeDatabase 2p: 4..."

Revert submission 19783737-presubmit-am-64cd2a18d36e4ed18e769469db7b10c6

Reason for revert: Crashing MP to recover DB takes more than 30s, whereas clearing DB and creating new schema takes less than 1s. 
Reverted Changes:
I16c0bf697:[automerge] Remove unused method downgradeDatabase...
Id3ec860e4:Remove unused method downgradeDatabase

Change-Id: Ieab3bd270dfb5d480b009f202b190b22c48043ae
1 file changed