Aw: Support simplified dark mode for pre-Q platform

WebView relies on android.R.attr.isLightTheme to detect if the
app in dark theme, it only works for Q+, this patch finds
isLightTheme resource by name for pre-Q platform, Android X uses
this attri for dark theme, it should work for most cases.

Bug: 1345249
Change-Id: I83fbd16bd9944e1fcd08473cb394fb9cd00e4a81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3782978
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Michael Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1027349}

android_webview/java/src/org/chromium/android_webview/DarkModeHelper.java

@@ -7,6 +7,7 @@
 import android.content.Context;
 import android.content.res.Configuration;
 import android.content.res.TypedArray;
+import android.os.Build;
 
 import androidx.annotation.VisibleForTesting;
 import androidx.core.graphics.ColorUtils;
@@ -67,8 +68,16 @@ public static int getNightMode(Context context) {
     public static int getLightTheme(Context context) {
         if (sLightThemeForTesting != null) return sLightThemeForTesting;
         int lightTheme = LightTheme.LIGHT_THEME_UNDEFINED;
-        TypedArray a =
-                context.getTheme().obtainStyledAttributes(new int[] {android.R.attr.isLightTheme});
+        int resId = android.R.attr.isLightTheme;
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.Q) {
+            // android.R.attr.isLightTheme is added in Q, for pre-Q platform, WebView
+            // checks if app has isLightTheme attr which could be added by Android X
+            // and wasn't stripped out.
+            resId = context.getApplicationContext().getResources().getIdentifier(
+                    "isLightTheme", "attr", context.getApplicationContext().getPackageName());
+            if (resId == 0) return lightTheme;
+        }
+        TypedArray a = context.getTheme().obtainStyledAttributes(new int[] {resId});
         if (a.hasValue(0)) {
             lightTheme = a.getBoolean(0, true) ? LightTheme.LIGHT_THEME_TRUE
                                                : LightTheme.LIGHT_THEME_FALSE;