TracingControllerAndroidImpl: Fix startup trace storage path on Android Q+

Migrate from Environment.getExternalStorageDirectory() to
Context.getExternalFilesDir() since the former no longer returns a
writable directory on Android Q. This change means traces are now still
written onto external media, but under a Chrome-specific directory.
Traces written to this directory can still be retrieved with adb,
including on non-rooted devices.

TEST=build/android/adb_profile_chrome_startup --browser=chromium --url=https://www.bbc.co.uk/

Bug: 1095587
Change-Id: I41cb8ec08137b362e012068a00eb71460d4bb4ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283597
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785913}