[Android] resetFlagsForTesting wipes cached flags from disk

CachedFeatureFlags#resetFlagsForTesting() resets sValuesReturned
in-memory already. This CL removes the values cached to disk.

This is irrelevant for non-batched tests, but for batched tests,
the first will cache values from native to disk, which will be
picked up in the next tests in the same batch. If the default
value of a flag in CachedFeatureFlags differs from the value
it has in the testing configuration, the first test in the batch may
get a different value than the other tests. This causes hidden flakes
because if a test failure happens, only the failed tests run, so
which test is the first in the batch varies with retries.

Bug: 1264165
Change-Id: If77803b1fd57d8ad29f3cfcee7e8526a0c9fc544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3252211
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#937002}