ui: Manage menu_ with std::vector.

Prior to this CL, menu_ is allocated with a fixed length of text_rows_.
However, because we support scrollable menu in wear_ui, there might be
more menu entries than text_rows_, which would lead to out-of-bounds
array access. This CL addresses the issue by switching to std::vector.

Bug: 65416558
Test: Run 'View recovery logs' on angler.
Test: Set large margin height that leaves text_rows less than 21. Then
      run 'View recovery logs' with 21 menu entries.
Change-Id: I5d4e3a0a097039e1104eda7d494c6269053dc894
(cherry picked from commit e15d7a51)