WIP: NSSKeyManager for logins component

[jo]

Add a NSSKeyManager for the Logins component. This enables the use of NSS for key management, like currently done in desktop.

Note: Currently there are no tests for the NSSKeyManager. This is due to the implicit nss initialization, resulting in nss being initialized without a profile by other tests.

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Branch builds: add [firefox-android: branch-name] to the PR title.