Eliminate scanner file cache

Get rid of the file cache, since it tends to run out of memory for large
numbers of files. This slows down the scanner somewhat, but recent
optimizations more than make up for that.

With this change, the postscan phase of the media scan now only processes
playlists. Removal of entries for files that no longer exist is done as
part of the prescan.
Lookups in the file cache are replaced by simple queries, which are still
reasonably fast because of a new index recently added to the media provider
database. Note that there was a bug in the case-insensitive matching for
file cache entries, in that e.g. an uppercase a-accent-aigue would be mapped
to its lowercase version, whereas the underlying case-insensitive filesystem
treats them as different characters. Getting rid of the file cache also fixes
this issue.

Bug: 4474617
Change-Id: I39c6f1a35bb518ef7ab912e9b9401663821ef48e