Add metrics logging for file operations.

Log to two new histograms - one for operations originating from system
providers, the other for operations originating from external providers.

Events in these histograms are bucketed by
- type (copy, move, delete, etc)
- destination (system vs external provider, with a special bucket for
    intra-provider operations)
- result (success vs failure)

BUG=26737132

Change-Id: Ic24c7a22fa3c1227e23cfbd42ae80ca1f70a19ba