Add backwards compatible implementation of overrideActivityTransition() (#4408)

This pull request adds `overrideActivityTransitionCompat()`, a
backwards-compatible version of `Activity.overrideActivityTransition()`
to be called in `Activity.onCreate()` in all Android versions.

This avoids duplicating the transition logic in different places of the
app to support older Android versions (in the activity launching code,
in `Activity.onCreate()` or in `Activity.finish()`).

- On API 34+, the implementation simply delegates to
`Activity.overrideActivityTransition()`.
- On API < 34, the implementation calls
`Activity.overridePendingTransition()` either immediately (opening
transition) or schedules it to be called later when the Activity is
finishing (closing transition).
- Rename `ActivityExensions.kt` to `ActivityExtensions.kt` (fix typo).

app/src/main/java/com/keylesspalace/tusky/util/ActivityExtensions.kt

@@ -0,0 +1,50 @@
+@file:JvmName("ActivityExtensions")
+
+package com.keylesspalace.tusky.util
+
+import android.app.Activity
+import android.content.Intent
+import android.os.Build
+import androidx.activity.ComponentActivity
+import androidx.annotation.AnimRes
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.LifecycleEventObserver
+import com.keylesspalace.tusky.BaseActivity
+
+fun Activity.startActivityWithSlideInAnimation(intent: Intent) {
+    // the new transition api needs to be called by the activity that is the result of the transition,
+    // so we pass a flag that BaseActivity will respect.
+    intent.putExtra(BaseActivity.OPEN_WITH_SLIDE_IN, true)
+    startActivity(intent)
+}
+
+/**
+ * Call this method in Activity.onCreate() to configure the open or close transitions.
+ */
+@Suppress("DEPRECATION")
+fun ComponentActivity.overrideActivityTransitionCompat(
+    overrideType: Int,
+    @AnimRes enterAnim: Int,
+    @AnimRes exitAnim: Int
+) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.UPSIDE_DOWN_CAKE) {
+        overrideActivityTransition(overrideType, enterAnim, exitAnim)
+    } else {
+        if (overrideType == ActivityConstants.OVERRIDE_TRANSITION_OPEN) {
+            overridePendingTransition(enterAnim, exitAnim)
+        } else {
+            lifecycle.addObserver(
+                LifecycleEventObserver { _, event ->
+                    if (event == Lifecycle.Event.ON_PAUSE && isFinishing) {
+                        overridePendingTransition(enterAnim, exitAnim)
+                    }
+                }
+            )
+        }
+    }
+}
+
+object ActivityConstants {
+    const val OVERRIDE_TRANSITION_OPEN = 0
+    const val OVERRIDE_TRANSITION_CLOSE = 1
+}