[roll] Roll fuchsia [ui][input-pipeline] rename files

Rename the source files to follow a consistent pattern:
1. Any source file defining a device binding is named `<device>_binding.rs`
2. Any source file defining a device handler is named `<device>_handler.rs`

Note that the bindings files usually contain addition types defining
events and device descriptors as well. It seems reasonable to me to
consider those part of the `<device>_binding` module.

Along the way
-------------
1. Rename <device_handler>::Handler to <device_handler>::<Device>Handler.
   This is also to improve consistency within this source directory.

   While we could go the other way (renaming everything to just
   <device_handler>::Handler), I think that the longer name is more
   consistent both within this subdirectory, and across fuchsia.git.

2. Rename touch_handler to gfx_touch_handler, to reflect the fact
   that it only works with the GFX protocol.

3. Make the documentation for the various TouchInjectorHandler
   constructors more uniform. (Some examples handled the Result
   with a '?', while others handled it with an `expect()`.)

Original-Bug: None
Test: fx build
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/602627
Original-Revision: 54dd75f3e5edb5de9101a49a53f2ec3aeac7ed0d
GitOrigin-RevId: 537b25f84432cf5b649701256f64315d2e812260
Change-Id: I8df29220d477c6001ba2ed987876ff5b3920b5f3