Bugfix: Add namespace to build.gradle to support gradle 8.x

[ducofronik]

Currently when updating our flutter projects to Gradle version 8.x we encountered the following error:

* What went wrong:
Failed to query the value of property 'buildFlowServiceProperty'.
> Could not isolate value org.jetbrains.kotlin.gradle.plugin.statistics.BuildFlowService$Parameters_Decorated@16aab65e of type BuildFlowService.Parameters
   > A problem occurred configuring project ':pushwoosh_flutter'.
      > Could not create an instance of type com.android.build.api.variant.impl.LibraryVariantBuilderImpl.
         > Namespace not specified. Specify a namespace in the module's build file. See https://d.android.com/r/tools/upgrade-assistant/set-namespace for information about setting the namespace.

           If you've specified the package attribute in the source AndroidManifest.xml, you can use the AGP Upgrade Assistant to migrate to the namespace value in the build file. Refer to https://d.android.com/r/tools/upgrade-assistant/agp-upgrade-assistant for general information about using the AGP Upgrade Assistant.

Cause: build.gradle is missing the namespace declaration in the android section.
Solution: Add namespace com.pushwoosh.plugin to build.gradle

[wfhm]

@ducofronik Thanks for submitting the PR!

The github repo of our flutter package is actually just a partial copy of the repo that maintains CI and publishing of the package, so merging this PR won't affect the original repo. We'll have to add this change internally first. We'll publish the next release with this fix and I'll update this thread shortly afterwards.