Cover StatisticsProvider with VPD unit tests.

Add unit tests for StatisticsProvider loading statistics from VPD echo
and VPD files.

Add the following test cases:
1. Basic loading for VPD echo and VPD files with valid and malformed
   values.
2. Recording of VPD histograms for missing and malofrmed VPD files.
3. Creating stub file when VPD file is missing on non-chromeos
   setup.

Bug: b:213325251
Test: chromeos_unittests --gtest_filter=StatisticsProviderImplTest.*
Change-Id: I4c0544e5854ee04ed80072a23add00024f39a1ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3768928
Reviewed-by: Victor-Gabriel Savu <vsavu@google.com>
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1049589}