Add target for ThemePicker instrumented tests

Changes are all to test code or visibility rules.

Bug: 249505965
Test: TreeHugger, manually compiled and ran picker
Change-Id: Ieb1b1d74f39c32e4a55ba625e5a476d027761045
diff --git a/Android.bp b/Android.bp
index d7fd50f..eb37963 100644
--- a/Android.bp
+++ b/Android.bp
@@ -62,7 +62,7 @@
 
     visibility: [
         ":__subpackages__",
-        "//packages/apps/ThemePicker",
+        "//packages/apps/ThemePicker:__subpackages__",
         "//vendor:__subpackages__",
     ],
 }