build(SettingsLib/SettingsThemeFlags): match minSdk with Android.bp

Otherwise it will inherit the minSdk=29 value from android settings
plugin declared in settings.gradle, which mismatches the SettingsTheme's
property (minSdk=21). The Gradle build passes without it but we get lint
errors.

Signed-off-by: iusmac <iusico.maxim@libero.it>

Author: iusmac

SettingsLib/SettingsThemeFlags/build.gradle

@@ -5,6 +5,10 @@ plugins {
 android {
     namespace 'com.android.settingslib.widget.theme.flags'
 
+    defaultConfig {
+        minSdk 21
+    }
+
     sourceSets {
         main {
             java.srcDirs = ['src']