Don't create cache when doing Apex PreRebootVerification

When StagingManager uses PackageParser2 to parse the APEX files during
pre-reboot verification with the parse flag PackageManager.GET_META_DATA,
it creates the caches of them. But, the caches are not used. Pass the
"null" instead of mCacheDir to avoid creating the caches.

Test: manual
Fix: 153618582
Change-Id: I40676082fc98d87ad2ad9fc94d02903afe0243d4