[XHG20033]

Summary

The Simplified Chinese translations were inherited from Google's TalkBack,
where the app is still called TalkBack, while the English source now says
Backtalk. This updates the display text to match:

Commit 1 — rebrand the display text (six values-zh-rCN files, 218 lines)

• Replace TalkBack/Talkback with Backtalk in the value text across the
  talkback module (strings.xml, compositor, keyboard tutorial, voice
  commands) and the braille modules (brailledisplay, brailleime), which
  all ship inside the same APK.
• Resource names and @string references (talkback_label etc.) are
  lowercase and intentionally untouched.
• Rewrite the TV training overview so Backtalk is the subject while
  Google's TalkBack stays as the origin: "Backtalk 是基于 Google 的
  TalkBack 打造的屏幕阅读器".
• The two voice command strings that launch TalkBack settings
  (voice_commands_talkback_settings / voice_commands_talkback_setting)
  are deliberately left as-is, since the English source still says
  "TalkBack" there.

Commit 2 — align a few strings with their English sources (5 lines)

A few inherited translations were longer than the current English source
(which had been shortened upstream) or mentioned the app name where the
English does not:

• bd_ime_disabled_dialog_message: drop the app name, matching "go to
  input settings to turn it on".
• pref_category_context_menu_summary and
  pref_category_context_menu_summary_single_finger: keep only the
  gesture instructions, as in English.
• title_pref_speech_footer: the English footer was reduced to the
  accessibility volume tip alone (upstream commit 9bcf0f5), so drop
  the stale speech rate paragraph.
• title_pref_talkback_speech_rate: "语速", matching the plain
  "Speech rate".

Commit 3 — rebrand the v33 and watch overlays (2 files, 6 occurrences)

The values-v33 and values-watch resource overlays have their own
values-zh-rCN counterparts, which the initial sweep missed: the
Android 13+ service intro and HTML description shown on the system
accessibility settings page, and the watch-only service description
and gesture-update notification.

Tested

• Builds cleanly with the existing CI configuration; no new strings and
  no placeholder changes, so the lint baseline is unaffected.
• AI involvement: the rename was performed by an AI-assisted scripted
  pass — candidate entries were found by a scanner that pairs each
  English string with its Simplified Chinese counterpart, the
  replacement was case-sensitive and touched value text only, and every
  changed line was verified to differ from the original by the brand
  word alone (a character-level diff check). The author of this PR
  reviewed the resulting diff before committing.
• The commit 2 rewordings were drafted by AI and reviewed and approved
  by the PR author line by line.
• Not exercised on device: the braille display strings (no braille
  hardware), and the v33 / watch overlay strings (no Android 13+ device
  and no watch). All of them are plain static text with no placeholders.