[jni] Fix jni Android Gradle space-assignment deprecations

[ancient0328]

This PR updates pkgs/jni/android/build.gradle to use explicit Groovy property assignment syntax.

Changed only pkgs/jni/android/build.gradle:

• group '...' -> group = '...'
• version '...' -> version = '...'
• namespace '...' -> namespace = '...'
• compileSdk 35 -> compileSdk = 35
• ndkVersion flutter.ndkVersion -> ndkVersion = flutter.ndkVersion
• minSdk 21 -> minSdk = 21

No Dart, Java, Kotlin, native runtime, public API, or dependency behavior is intended to change.

This follows Gradle's upgrade guide for Groovy space-assignment deprecations, which are scheduled for removal in Gradle 10.

Related issue: #3347

Reference: https://docs.gradle.org/current/userguide/upgrading_version_8.html#groovy_space_assignment_syntax

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[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	None	1.0.0	1.0.1-wip	1.0.1-wip	✔️

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources
jni	$JIterator	core_bindings.dart::JIterator::implementIn::$impl core_bindings.dart::JIterator::implement::$impl
jni	$JCollection	core_bindings.dart::JCollection::implementIn::$impl core_bindings.dart::JCollection::implement::$impl
jni	$JList	core_bindings.dart::JList::implementIn::$impl core_bindings.dart::JList::implement::$impl
jni	$JMap$JEntry	core_bindings.dart::JMap$JEntry::implementIn::$impl core_bindings.dart::JMap$JEntry::implement::$impl
jni	$JMap	core_bindings.dart::JMap::implementIn::$impl core_bindings.dart::JMap::implement::$impl
jni	$JSet	core_bindings.dart::JSet::implementIn::$impl core_bindings.dart::JSet::implement::$impl

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[liamappelbe]

Looks fine to me, but you'll need to sign the CLA