fixup! Keyguard: Add option to scramble pin layout when unlocking (2/2).

Set the content description whenever the digit is updated, not just
once when the key is created.

Test: Manual: Review output created by `adb shell uiautomator dump`
      to ensure text of inner node matches content description of
      outer node for keys, and/or verify when navigating between
      keys with TalkBack.
Issue: calyxos#3279
Change-Id: I4be8ad9777f17e9b7f98352e4961a0038eb4fb5b