fix(android): adjust for Kotlin and project warnings

Committed via a GitHub action: https://github.com/tauri-apps/plugins-workspace/actions/runs/9307017436

Co-authored-by: lucasfernog <lucasfernog@users.noreply.github.com>

android/build.gradle.kts

@@ -8,7 +8,7 @@ android {
     compileSdk = 34
 
     defaultConfig {
-        minSdk = 19
+        minSdk = 24
         targetSdk = 34
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

android/src/main/java/ShellPlugin.kt

@@ -21,7 +21,7 @@ class ShellPlugin(private val activity: Activity) : Plugin(activity) {
             val url = invoke.parseArgs(String::class.java)
             val intent = Intent(Intent.ACTION_VIEW, Uri.parse(url))
             intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
-            activity?.applicationContext?.startActivity(intent)
+            activity.applicationContext?.startActivity(intent)
             invoke.resolve()
         } catch (ex: Exception) {
             invoke.reject(ex.message)