Support expiration of preloaded file cache

When device is provisioned, we delete all files from /data/preloads
except file_cache. We should do best effort to keep file_cache during
the first config_keepPreloadsMinDays. After that,
persist.sys.preloads.file_cache_expired is set to 1, which indicates
that cache can be deleted when additional storage space is requested.

Bug: 34690396
Test: Manual + RetailDemoModeServiceTest
Change-Id: Ie584a9dd6689bcc5e6b3cb448e95dfe5f73d2eeb