fs_mgr: refactor pre-mount logic into prepare_fs_for_mount()

There were several duplications in the code that runs before a
filesystem is mounted.  This made it difficult to start running tune2fs
to set the encryption feature flag.  Refactor to deduplicate the logic,
and improve the log messages.

Bug: 36231741
Change-Id: I90846dad9c5ec85b3c5460615dec4cc19cb7e198