Integrate cert4android as an android library (bitfireAT/davx5#274)

* Removed `cert4android` submodule

Signed-off-by: Arnau Mora Gras <arnyminerz@proton.me>

* Added cert4android dependency

Signed-off-by: Arnau Mora Gras <arnyminerz@proton.me>

* Update dependencies, fix invalid style

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Signed-off-by: Arnau Mora Gras <arnyminerz@proton.me>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>