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 45 seconds (queued for 2 seconds)
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librem5
build:native

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librem5
build-gtkdoc

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librem5
check-po

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librem5
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librem5
package-pureos-byzantium

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librem5:arm64
package-pureos-byzantium:arm64

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librem5
autopkgtest-pureos-byzantium-package

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librem5
lintian-pureos-byzantium-package

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