credentials: Rework loading credentials

This gets rid of the `calls_credentials_update_from_keyfile()`, which had to be
used after `calls_credentials_new()` in favour of
`calls_credentials_new_from_keyfile()`.

Additionally it introduces `calls_credentials_save_to_key_file()`
8 jobs for wip/ui-manage-accounts in 10 minutes and 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #353622
librem5
build:native

00:01:59

 
  Test
passed #353624
librem5
build-gtkdoc

00:02:05

passed #353625
librem5
check-po

00:00:25

passed #353623
librem5
test:native

00:02:03

24.4%
 
  Package
passed #353626
librem5
package-pureos-byzantium

00:02:04

passed #353627
librem5:arm64
package-pureos-byzantium:arm64

00:06:26

 
  Test Package
passed #353628
librem5
autopkgtest-pureos-byzantium-package

00:01:14

passed #353629
librem5
lintian-pureos-byzantium-package

00:00:41