Flush UMA session on pause

We don't want to end the UMA session because we might be starting an
Activity in the same session (eg. History), but we still need to flush
the existing record and state as Android might kill us before we get
stopped.

We also kill ourselves when setting chrome://flags before stop is
called, so that also necessitates a flush.

Flagged by UmaSessionsCorrectnessFixes.

Bug: 441405026, 400975101
Change-Id: Ic871df1077a54231e55d2b8199dcf37dc38a62d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6931621
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1514656}
NOKEYCHECK=True
GitOrigin-RevId: 2adfd07d057d8a363cf814a33aa8182b1ccaceb2