Use Proguard rules for Thunderbird release builds

[cketti]

Currently the release build of :app-thunderbird doesn't include the proguard-rules.pro file. This leads to R8 stripping too much code and the app crashing when setting up an IMAP account.

Steps to reproduce:

1. ./gradlew :app-thunderbird:assembleRelease (make sure a keystore is available so a signed APK is created)
2. adb install app-thunderbird/build/outputs/apk/release/app-thunderbird-release.apk
3. Set up an IMAP account

Stack trace:

java.lang.AssertionError: Missing field in com.fsck.k9.mail.Flag
    at com.squareup.moshi.StandardJsonAdapters$EnumJsonAdapter.<init>(SourceFile:92)
    at com.squareup.moshi.MapJsonAdapter$1.create(SourceFile:514)
    at com.squareup.moshi.Moshi.adapter(SourceFile:29)
    at com.squareup.moshi.MapJsonAdapter$1.create(SourceFile:1069)
    at com.squareup.moshi.Moshi.adapter(SourceFile:29)
    at com.fsck.k9.controller.PendingCommandSerializer.<init>(SourceFile:102)
    at com.fsck.k9.controller.PendingCommandSerializer.<clinit>(SourceFile:3)
    at com.fsck.k9.mailstore.LocalStore.<init>(SourceFile:6)
    at com.fsck.k9.mailstore.LocalStoreProvider.getInstance(SourceFile:48)
    at com.fsck.k9.mailstore.SpecialLocalFoldersCreator.createSpecialLocalFolders(SourceFile:13)
    at com.fsck.k9.account.AccountCreator$createAccount$2.invokeSuspend(SourceFile:485)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(SourceFile:9)
    at kotlinx.coroutines.DispatchedTask.run(SourceFile:113)
    at androidx.work.Worker$2.run(SourceFile:13)
    at kotlinx.coroutines.scheduling.TaskImpl.run(SourceFile:3)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(SourceFile:96)
    Suppressed: kotlinx.coroutines.internal.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@bfda7dd, Dispatchers.Main.immediate]
Caused by: java.lang.NoSuchFieldException: RECENT
    at java.lang.Class.getField(Class.java:2337)
    at com.squareup.moshi.StandardJsonAdapters$EnumJsonAdapter.<init>(SourceFile:33)
    ... 15 more