For #10948 - Avoid PWA onboarding if user already has them on screen

It is possible that after migration users would already have Firefox PWAs on
their screen.
Since they already know about this functionality, we should not promote it to
them with the one-off `FirstTimePwaFragment`.

To query installed PWAs we'll use an API available only on Android >= 26 which
means that we will probably have half of users with PWAs still see the
onboarding but half which will not.

app/src/main/java/org/mozilla/fenix/utils/Settings.kt

@@ -9,6 +9,8 @@ import android.app.Application
 import android.content.Context
 import android.content.Context.MODE_PRIVATE
 import android.content.SharedPreferences
+import android.content.pm.ShortcutManager
+import android.os.Build
 import android.view.accessibility.AccessibilityManager
 import androidx.annotation.VisibleForTesting
 import androidx.annotation.VisibleForTesting.PRIVATE
@@ -454,10 +456,34 @@ class Settings private constructor(
         default = false
     )
 
-    var shouldShowFirstTimePwaFragment by booleanPreference(
-        appContext.getPreferenceKey(R.string.pref_key_show_first_time_pwa),
-        default = true
-    )
+    var shouldShowFirstTimePwaFragment: Boolean
+        get() {
+            val alreadyShownPwaOnboarding = preferences.getBoolean(
+                appContext.getPreferenceKey(R.string.pref_key_show_first_time_pwa), false)
+
+            // ShortcutManager::pinnedShortcuts is only available on Oreo+
+            if (!alreadyShownPwaOnboarding && Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+                val alreadyHavePWaInstalled =
+                    appContext.getSystemService(ShortcutManager::class.java)
+                        .pinnedShortcuts.size > 0
+
+                // Users don't need to be shown the PWA onboarding if they already have PWAs installed.
+                preferences.edit()
+                    .putBoolean(
+                        appContext.getPreferenceKey(R.string.pref_key_show_first_time_pwa),
+                        alreadyHavePWaInstalled)
+                    .apply()
+
+                return !alreadyHavePWaInstalled
+            }
+
+            return !alreadyShownPwaOnboarding
+        }
+        set(value) {
+            preferences.edit()
+                .putBoolean(appContext.getPreferenceKey(R.string.pref_key_show_first_time_pwa), value)
+                .apply()
+        }
 
     @VisibleForTesting(otherwise = PRIVATE)
     internal val trackingProtectionOnboardingCount by intPreference(