Regenerate ThemeHostTest reference images with local system image

Implement a "local" option in generate_images.py, which runs emulator
without a SDK and it is using a locally build system image.
Generate new reference images for ThemeHostTest to resolve differences
in antialiased pixels. Images were generated using a system image from
a local build synced with a manifest file of ab/4474236.

Bug: 67866323
Test: passed ThemeHostTest.
Change-Id: I74d478379573461310a9f3567ec87add089eb079
diff --git a/hostsidetests/theme/generate_images.py b/hostsidetests/theme/generate_images.py
index c73457a..5dcc76f 100755
--- a/hostsidetests/theme/generate_images.py
+++ b/hostsidetests/theme/generate_images.py
@@ -181,7 +181,10 @@
 
 
 def start_emulator(name, density):
-    emu_path = get_emulator_path()
+    if name == "local":
+        emu_path = ""
+    else:
+        emu_path = get_emulator_path()
 
     # Start emulator for 560dpi, normal screen size.
     test_avd = AVD(name, emu_path)