Add statistics logging for MediaProvider.

As part of becoming a Mainline module, we need to understand basic
information about the health of our module in the wild.  For the
MediaProvider module, we're interested in identifying these cases:

-- When scan operations result in an unexpectedly large number of
inserts, updates, or deletes in proportion to the total number of
files indexed.  This typically indicates user data loss or a missing
database upgrade step.

-- When the overall duration of scan operations becomes significantly
slower in relation to the number of files indexed.  This typically
indicates an indexing performance regression.

-- When a scan operation skips over an entire directory tree.  This
can indicate an app placing ".nomedia" files in unexpected locations.

-- When a large number of media files are deleted by a specific
app.  This can help identify data loss bugs caused by MediaProvider
directly, or attribute data loss bugs in other apps.

-- When database upgrade/downgrade operations take a substantial
amount of time in proportion to the total number of files.  This
typically indicates a performance regression.

-- When performing idle maintenance, a large number of stale or
expiring media can indicate an invalidation bug.

Bug: 143723019
Test: manual
Change-Id: I89c5b5b51a843a67348a7bb4b8e6ac01fb2b15b9