Throttle UI actions instead of debouncing (#3651)

Introduce Flow<T>.throttleFirst(). In a flow this emits the first value,
and each value afterwards that is > some timeout after the previous
value.

This prevents accidental double-taps on UI elements from generating
multiple-actions.

The previous code used debounce(). That has a similar effect, but with
debounce() the code has to wait until after the timeout period has
elapsed before it can process the action, leading to an unnecessary
UI delay.

With throttleFirst a value is emitted immediately, there's no need
to wait. It's subsequent values that are potentially throttled.

app/src/test/java/com/keylesspalace/tusky/util/FlowExtensionsTest.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2023 Tusky Contributors
+ *
+ * This file is a part of Tusky.
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * Tusky is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+ * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with Tusky; if not,
+ * see <http://www.gnu.org/licenses>.
+ */
+
+package com.keylesspalace.tusky.util
+
+import app.cash.turbine.test
+import com.google.common.truth.Truth.assertThat
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.test.advanceUntilIdle
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+import kotlin.time.Duration.Companion.milliseconds
+import kotlin.time.ExperimentalTime
+
+@OptIn(ExperimentalCoroutinesApi::class, ExperimentalTime::class)
+class FlowExtensionsTest {
+    @Test
+    fun `throttleFirst throttles first`() = runTest {
+        flow {
+            emit(1) // t = 0, emitted
+            delay(90.milliseconds)
+            emit(2) // throttled, t = 90
+            delay(90.milliseconds)
+            emit(3) // throttled, t == 180
+            delay(1010.milliseconds)
+            emit(4) // t = 1190, emitted
+            delay(1010.milliseconds)
+            emit(5) // t = 2200, emitted
+        }
+            .throttleFirst(1000.milliseconds, timeSource = testScheduler.timeSource)
+            .test {
+                advanceUntilIdle()
+                assertThat(awaitItem()).isEqualTo(1)
+                assertThat(awaitItem()).isEqualTo(4)
+                assertThat(awaitItem()).isEqualTo(5)
+                awaitComplete()
+            }
+    }
+}