[PE File Vars] Updated file expiry logic

[nishant-clevertap]

• The file expiry timestamp were not getting updated for PE File vars if file was already present because we were only updating the expiry timestamp when downloadFiles: is called. For InApps downloadFiles: are called for all urls, but for PE File Var we are calling it for urls which are not present.
• The expired files were not getting deleted if we don't call downloadFiles: anywhere. So added logic to check for expired files and delete them when CTFileDownloader is initialised only, ie on every App launch.

[nzagorchev]

LGTM