feat(android): settings as a page per area, and a language picker

The first Settings page lists Playback, Downloads and storage,
Appearance, Metadata and About, each with a line saying where it stands,
and each opens its own page with a way back. On one long page, every
setting added pushed the rest further from reach.

Audio and subtitle language are chosen from a list of languages by name
instead of typed as "jpn or eng". The list stores the three-letter tag a
Matroska file carries, reads a stored tag in either ISO form, and still
takes any other tag typed in.

android/app/src/test/java/io/narl/protonstream/ui/LanguagePickerTest.kt

@@ -0,0 +1,31 @@
+package io.narl.protonstream.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class LanguagePickerTest {
+    @Test
+    fun `a stored tag reads as its language in either ISO form`() {
+        assertEquals("Japanese", languageLabel("jpn"))
+        assertEquals("Japanese", languageLabel("ja"))
+        assertEquals("German", languageLabel("deu"))
+        assertEquals("German", languageLabel("ger"))
+    }
+
+    @Test
+    fun `a region suffix and capitals do not hide the language`() {
+        assertEquals("Portuguese", languageLabel("PT-br"))
+    }
+
+    @Test
+    fun `a tag not on the list is shown as typed, and blank is no preference`() {
+        assertEquals("tgl", languageLabel("tgl"))
+        assertEquals("No preference", languageLabel(null))
+        assertEquals("No preference", languageLabel("  "))
+    }
+
+    @Test
+    fun `every listed language has a two-letter form`() {
+        LANGUAGES.forEach { (code, name) -> assertEquals(code, name, languageLabel(code)) }
+    }
+}

android/app/src/test/java/io/narl/protonstream/ui/ScreenshotTest.kt

@@ -108,6 +108,16 @@ class ScreenshotTest {
         SettingsScreen(LIBRARY, { _, _, _, _ -> }, {}, {}, {}, PaddingValues())
     }
 
+    @Test
+    fun `the playback settings`() = capture("settings-playback") {
+        SettingsScreen(LIBRARY, { _, _, _, _ -> }, {}, {}, {}, PaddingValues(), SettingsPage.Playback)
+    }
+
+    @Test
+    fun `the storage settings`() = capture("settings-storage") {
+        SettingsScreen(LIBRARY, { _, _, _, _ -> }, {}, {}, {}, PaddingValues(), SettingsPage.Storage)
+    }
+
     @Test
     fun `the history page`() = capture("history-phone") {
         // Fixed day and zone: "Today" and "Yesterday" are relative, and an