Rename BACK_ALT flag to BACK_DISMISS_IME

The NAVIGATION_HINT_BACK_ALT flag was originally introduced in [1],
alongside a few other flags which affected the navigation bar buttons.
The other flags were eventually removed in [2], leaving only this.
Unfortunately, the flag name is not very explicit, and it lacks clear
documentation.

This redefines the flag as BACK_DISMISS_IME, representing whether the
back button is visually adjusted to indicate that it will dismiss the
IME when pressed.

  [1]: Ie64369e3df3178178f35b56763ea96d380b6ddd4
  [2]: Ifd1f8c9f400d90542f0ca858b9a4deacabbd518a

Flag: EXEMPT refactor
Bug: 366129400
Test: n/a
Change-Id: I90b8cecaad879c2defc3e18711d5d4761bf64671