bug(android/engine): Download intent requires FLAG_IMMUTABLE for Android S+

[sentry-io]

Crash reported from user testing of #7832
#7832 (comment)

RuntimeException: Error receiving broadcast Intent { act=android.intent.action.DOWNLOAD_COMPLETE flg=0x10 pkg=com.t...

Sentry Issue: KEYMAN-ANDROID-288

IllegalArgumentException: com.tavultesoft.kmapro.debug: Targeting S+ (version 31 and above) requires that one of FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a PendingIntent.
Strongly consider using FLAG_IMMUTABLE, only use FLAG_MUTABLE if some functionality depends on the PendingIntent being mutable, e.g. if it needs to be used with inline replies or bubbles.
    at android.app.PendingIntent.checkFlags(PendingIntent.java:375)
    at android.app.PendingIntent.getActivitiesAsUser(PendingIntent.java:593)
    at android.app.PendingIntent.getActivities(PendingIntent.java:575)
    at android.app.TaskStackBuilder.getPendingIntent(TaskStackBuilder.java:275)
    at android.app.TaskStackBuilder.getPendingIntent(TaskStackBuilder.java:249)
...
(19 additional frame(s) were not displayed)

RuntimeException: Error receiving broadcast Intent { act=android.intent.action.DOWNLOAD_COMPLETE flg=0x10 pkg=com.tavultesoft.kmapro.debug (has extras) } in com.tavultesoft.kmea.cloud.CloudDownloadMgr$1@562b474
    at android.app.LoadedApk$ReceiverDispatcher$Args.lambda$getRunnable$0$LoadedApk$ReceiverDispatcher$Args(LoadedApk.java:1689)
    at android.app.LoadedApk$ReceiverDispatcher$Args$$ExternalSyntheticLambda0.run
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loopOnce(Looper.java:201)
...
(5 additional frame(s) were not displayed)

[darcywong00]

Maybe need to also 🍒 pick to stable-15.0

[darcywong00]

Fixed in 16.0 beta by #7844