settings: snap the preamp to its grid and drop a duplicate header

Review follow-ups.

- Remove the second "Playback" section header the rows were added
  under; the section already had one, so the screen rendered it twice.
- Snap the preamp to the 0.5 dB grid in the dialog. Material's slider
  snaps in pixel space and converts back, so the value it reports is near
  the configured grid but not on it: -1.0 dB arrives as -0.99999905, was
  displayed verbatim and was stored that way until the next drag. The
  step count is now derived from the range rather than hardcoded.
- Format the decibel value with the resource locale so translated
  layouts get their own decimal separator.
- The controls move to `ReplayGainControls.kt`, since the file now holds
  the grid as well as the labels, and the snap is covered by a test that
  reproduces the off-grid value without it.

app/src/test/kotlin/com/subtracks/ui/settings/ReplayGainControlsTest.kt

@@ -3,13 +3,15 @@ package com.subtracks.ui.settings
 import android.content.Context
 import androidx.test.core.app.ApplicationProvider
 import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.subtracks.data.prefs.MAX_REPLAY_GAIN_PREAMP_DB
+import com.subtracks.data.prefs.MIN_REPLAY_GAIN_PREAMP_DB
 import com.subtracks.data.prefs.ReplayGainMode
 import org.junit.Assert.assertEquals
 import org.junit.Test
 import org.junit.runner.RunWith
 
 @RunWith(AndroidJUnit4::class)
-class ReplayGainLabelsTest {
+class ReplayGainControlsTest {
     private val resources = ApplicationProvider.getApplicationContext<Context>().resources
 
     @Test
@@ -25,4 +27,20 @@ class ReplayGainLabelsTest {
         assertEquals("-2.5 dB", replayGainPreampLabel(resources, -2.5f))
         assertEquals("0.0 dB", replayGainPreampLabel(resources, 0f))
     }
+
+    @Test
+    fun theSliderHasOneStepPerHalfDecibel() {
+        assertEquals(60, ((MAX_REPLAY_GAIN_PREAMP_DB - MIN_REPLAY_GAIN_PREAMP_DB) / REPLAY_GAIN_PREAMP_STEP_DB).toInt())
+        assertEquals(59, REPLAY_GAIN_PREAMP_STEPS)
+    }
+
+    @Test
+    fun thePreampSnapsBackOntoTheHalfDecibelGrid() {
+        assertEquals(0f, snapReplayGainPreampDb(-0f), 0f)
+        assertEquals(-1f, snapReplayGainPreampDb(-0.99999905f), 0f)
+        assertEquals(-3f, snapReplayGainPreampDb(-3.000001f), 0f)
+        assertEquals(14.5f, snapReplayGainPreampDb(14.6f), 0f)
+        assertEquals(MAX_REPLAY_GAIN_PREAMP_DB, snapReplayGainPreampDb(14.9f), 0f)
+        assertEquals(MIN_REPLAY_GAIN_PREAMP_DB, snapReplayGainPreampDb(-14.9f), 0f)
+    }
 }