android: drop P screen size to 800x480.

A few tests are surprisingly sensitive to screen size.

Bug: 1030919
Change-Id: I2bdf22dc99e6b02041bc10998140bf0c8491e655
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1998706
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731578}

tools/android/avd/proto/generic_android28.textpb

@@ -23,3 +23,11 @@ avd_package {
   dest_path: ".android"
 }
 avd_name: "android_28_google_apis_x86"
+
+avd_settings {
+  screen {
+    density: 160
+    height: 800
+    width: 480
+  }
+}