Add new Theme "Use System Design" + fixes to night mode (#1069)

* Add theme system

A theme which follows system design.
See: https://www.xda-developers.com/samsung-galaxy-s9-update-night-mode-schedule/

* update

to be in line with https://github.com/tuskyapp/Tusky/pull/1060/files

* Update ThemeUtils.java

* update

* Cleanup

* Update Deps

* Cleanup

* Update PreferencesActivity.kt

* Workaround to make MODE_NIGHT_FOLLOW_SYSTEM work

* Update ThemeUtils.java

* Use ThemeUtils.THEME_SYSTEM

* Update SplashActivity.kt

* Update strings.xml

* Update Deps

* Update build.gradle

* Update build.gradle

* fix tests

app/src/test/java/com/keylesspalace/tusky/ComposeActivityTest.kt

@@ -26,6 +26,7 @@ import com.keylesspalace.tusky.entity.Account
 import com.keylesspalace.tusky.entity.Emoji
 import com.keylesspalace.tusky.entity.Instance
 import com.keylesspalace.tusky.network.MastodonApi
+import com.keylesspalace.tusky.util.ThemeUtils
 import okhttp3.Request
 import okhttp3.ResponseBody
 import org.junit.Assert
@@ -44,6 +45,7 @@ import retrofit2.Call
 import retrofit2.Callback
 import retrofit2.Response
 
+
 /**
  * Created by charlag on 3/7/18.
  */
@@ -55,6 +57,7 @@ class ComposeActivityTest {
     lateinit var activity: ComposeActivity
     lateinit var accountManagerMock: AccountManager
     lateinit var apiMock: MastodonApi
+    lateinit var themeUtilsMock: ThemeUtils
 
     val account = AccountEntity(
             id = 1,
@@ -135,9 +138,12 @@ class ComposeActivityTest {
         val dbMock = mock(AppDatabase::class.java)
         `when`(dbMock.instanceDao()).thenReturn(instanceDaoMock)
 
+        themeUtilsMock = Mockito.mock(ThemeUtils::class.java)
+
         activity.mastodonApi = apiMock
         activity.accountManager = accountManagerMock
         activity.database = dbMock
+        activity.themeUtils = themeUtilsMock
 
         `when`(accountManagerMock.activeAccount).thenReturn(account)