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Now that we're offering to store private app data on adopted storage
devices, the performance of those devices is much more important to
overall user experience.

To help set user expectations, this change offers to execute a
real-world benchmark on a storage device, returning a metric that can
be used to compare internal and external storage.  The benchmark is
generated from the strace-instrumented storage access patterns of
typical apps.

A typical device completes the benchmark in under two seconds on
internal storage, a UHS-3 SD card is even faster (!), but a very slow
Class 4 SD card takes about 30 seconds to complete, giving us a clear
signal.

The measured benchmark numbers are logged along with information
about the storage device, such as manufacturer, model, etc.  Card
serial numbers are scrubbed from output.

Bug: 21172095
Change-Id: I9b2713dafdfdfcf5d97bf1bc21841f39409a7e54