Replace InputWindowInfo#inputChannel with an IBinder token.

The IBinder token is now being used as the UUID for InputWindows.
We can pass it around without the channel to avoid unnecessary FD
parcelling, duping, and other juggling.

Test: Existing tests pass.
Bug: 80101428
Bug: 113136004
Bug: 111440400
Change-Id: I8eba3fa05f249b7dfcb5c3d9817241cbfe9ab76c