Add atom for general external storage access stats

Define a new pulled atom which will be logged in MediaProvider.
The atom collects general access stats for external storage:
* Total number of accesses
* Total number of file path accesses
* Total number of secondary storage accesses
* List of accesses mime types

Test: build
Test: make statsd_testdrive ; ./out/host/linux-x86/bin/statsd_testdrive \
      -p com.google.android.providers.media.module 10085
Bug: 149669087
Change-Id: Ic0ebb040a05ec06603f1a617f290af6a2f21916d