feat: allow build-time overrides of attestation server url and static fingerprint override

[cassandracomar]

pulls the attestation server url and overriden fingerprint from res/values/strings, allowing them to be overriden at AOSP/GrapheneOS build time. because the device info/fingerprint can't be known statically, this PR introduces a dynamic look up for the device info and fingerprint via a singleton that gets its context set by the main activity, avoiding null pointer issues by ensuring static accesses for Resources in AttestationProtocol.java occur as late as possible.

[muhomorr]

Turning RemoteVerifyJob.DOMAIN constant into a BuildConfig constant would allow to simplify these changes a lot. See Apps app for an example: https://github.com/GrapheneOS/Apps/blob/9bb136db0aacfbaef8c5ffe3c8980eb582087dba/app/build.gradle.kts#L50 https://github.com/GrapheneOS/Apps/blob/9bb136db0aacfbaef8c5ffe3c8980eb582087dba/app/src/main/java/app/grapheneos/apps/core/RepoRetriever.kt#L26