recovery: Fix the bug that truncates menu entries.

When there are 20 entries (like 10 last_log* and 10 last_kmg* in "view
recovery logs"), there's no "Back" entry. Because the number of entries
(21) exceeds text_rows (20) in WearRecoveryUI::StartMenu(). Since we
have scrollable menu, having more entries than text_rows won't be an
issue.

Bug: 23752519
Change-Id: I12573d7a34852a1a3d130c9e88522cee737eb08f