[TTFPassGen] Show the bottom sheet

Introduces the MVC infrastructure for the password generation bottom
sheet. Method `TouchToFillPasswordGenerationCoordinator.show()` displays
the bottom sheet.
Accessibility strings added to announce the bottom sheet opening and
closing (see the mocks
https://bugs.chromium.org/p/chromium/issues/detail?id=1421753#c10).

Bug: 1421753
Change-Id: I7f0c66c8afe4c524a2f3cb04380db2602340005e
Low-Coverage-Reason: There is very little functionality in the TouchToFillPasswordGenerationBridge (it just delegates the calls to the coordinator) and in TouchToFillPasswordGenerationView (it just implements the interface's methods).
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4521118
Commit-Queue: Anna Tsvirchkova <atsvirchkova@google.com>
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148419}