[iOS][BWG] Add BWG first prompt submission metrics.

Add MobileGeminiFirstPromptSubmitted user action and
IOS.Gemini.FirstPrompt.SubmissionMethod histogram for first prompt
submission to track how users submit their first prompt per BWG session.

Bug: 435649967
Change-Id: I2d6124b0eff476e6a1a12587c682ce39b3afc00a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6825206
Auto-Submit: Ibrahim Kanouche <kanouche@google.com>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Adam Arcaro <adamta@google.com>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1498736}