fix(proguard): update rules to preserve enum constants and collection interfaces for Moshi

[rafaeltonholo]

Fix #10697.

This pull request updates the ProGuard rules for both the K-9 Mail and Thunderbird Android apps to ensure compatibility with Moshi's JSON serialization/deserialization, especially when using R8 optimizations. The changes prevent R8 from stripping necessary enum constants and class structures that Moshi relies on for reflective access.

Changes

• Added a rule to keep all fields in the net.thunderbird.core.common.mail.Flag enum, preventing R8 from stripping enum constants required by Moshi's EnumJsonAdapter for JSON deserialization.
• Added a rule to keep all inner classes and fields of com.fsck.k9.controller.MessagingControllerCommands, ensuring Moshi can access collection types via reflection and is not broken by R8's class rewriting.

Testing

• The test must be performed on release variants: K-9 fossRelease, Thunderbird foss/fullDaily, foss/fullBeta, foss/fullRelease
• Test both adding a new account and importing accounts on both the K-9 and Thunderbird apps.

[wmontwe]

LGTM