android: Set launchMode="singleTask" instead of "singleTop"

[chrisbobbe]

This addresses #620 by making it so the whole app state isn't reset when coming back from Firefox in the web-auth flow. Discussion:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/Flutter.20app.20.2B.20SAML.20auth/near/1776828

Later in that discussion, Greg found that this mode is the most appropriate for our app, and that setting taskAffinity="" is probably a good idea as a security-hardening measure.

Fixes: #620

[gnprice]

Thanks for debugging and fixing this!

Let's make this two commits: first taskAffinity, which is an independent hardening measure, then launchMode. Otherwise all LGTM.

[chrisbobbe]

Cool; done, PTAL.

[gnprice]

Thanks! Looks good; merging, with two commit-message tweaks. One is a minor formatting change to make a paragraph wrap better, and the other a small correction or clarification:

$ git range-diff --creation-factor=90 origin pr/639 @
1:  7588b5a47 ! 1:  138b1e28b android: Set taskAffinity=""
    @@ Commit message
         android: Set taskAffinity=""
     
         This looks like a security-hardening measure that would be good to
    -    do, independently of the upcoming change to
    -    `launchMode="singleTask"`. Discussion:
    +    do, independently of the upcoming change to launchMode="singleTask".
    +    Discussion:
           https://chat.zulip.org/#narrow/stream/48-mobile/topic/Flutter.20app.20.2B.20SAML.20auth/near/1790071
     
      ## android/app/src/main/AndroidManifest.xml ##
2:  1e9a27d1f ! 2:  1251a0b06 android: Set launchMode="singleTask" instead of "singleTop"
    @@ Commit message
         This addresses #620 by making it so the whole app state isn't reset
         when coming back from Firefox in the web-auth flow. Discussion:
           https://chat.zulip.org/#narrow/stream/48-mobile/topic/Flutter.20app.20.2B.20SAML.20auth/near/1776828
    +
         Later in that discussion, Greg found that this mode is the most
    -    appropriate for our app.
    +    appropriate for the main activity of our app:
    +      https://chat.zulip.org/#narrow/stream/48-mobile/topic/Flutter.20app.20.2B.20SAML.20auth/near/1790019

         Fixes: #620

In particular the key about singleTask is that it's appropriate for the "main" or "root" activity of an app. We have some ancillary activities in our merged manifest, like this one (which I guess would get used if we passed [LaunchMode.inAppWebView] to launchUrl):

        <activity
            android:theme="@ref/0x1030007"
            android:name="io.flutter.plugins.urllauncher.WebViewActivity"
            android:exported="false" />

and those wouldn't be appropriate for android:launchMode="singleTask".

[chrisbobbe]

I see; interesting. Thanks!