Keypad: Do not show allow typing + when only_digits is true

The keypad shouldn't allow typing or show + when only_digits
is set to true. Therefore this adds the correct behavior.
7 jobs for keypad-zero in 12 minutes and 45 seconds (queued for 6 minutes and 18 seconds)
latest
Status Job ID Name Coverage
  Build
passed #234218
librem5
build-debian-clang

00:04:01

passed #234216
librem5
build-debian-gcc

00:02:57

passed #234217
librem5
build-debian-gcc-static

00:02:48

passed #234219
librem5
build-fedora-gcc

00:03:28

 
  Docs
passed #234220
librem5
build-gtkdoc

00:02:03

 
  Test
passed #234221
librem5
unit-test

00:01:38

44.1%
 
  Package
passed #234222
librem5
package-debian

00:02:03