Update Saber to v1.34.1, build pdfium from source

Required

• The app complies with the inclusion criteria
• All related fdroiddata and RFP issues have been referenced in this merge request
• Builds with fdroid build

Strongly Recommended

• The upstream app source code repo contains the app metadata (summary/description/images/changelog/etc) in a Fastlane or Triple-T folder structure
• Releases are tagged

Suggested

• External repos are added as git submodules instead of srclibs
• Multiple apks for native code

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Closes !37930 (closed)

Closes #3926 (closed)