This patch fixes that Classifier::ActiveTables doesn't return v4 tables.
Classifier::mActiveTablesCache is generated by scanning safebrowsing directory.
We use Classifier::ScanStoreDir to do the work, but it will ignore subdirectory.
Since v4 tables are stored in subdirectory 'google4', mActiveTablesCache doesn't
include v4 tables.
Fix this issue by checking subdirectory recursively in ScanStoreDir.
MozReview-Commit-ID: I6pa6e4bFND