Add is_android_appliance

Add support for internal builds of non-standard android
similar to is_android_things, but not including changes
specific to that platform build. For use mainly in
internal but also to generalize a few non-internal
current uses.

Bug: n/a
Test: build, initial use in internal
Change-Id: I17099b61a34f020142008f25d5df05d68f653c7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275368
Auto-Submit: Brett Kilty <brettk@google.com>
Commit-Queue: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783976}

chromecast/chromecast.gni

@@ -53,6 +53,9 @@ declare_args() {
   # Set true for builds targeting Android Things.
   is_android_things = false
 
+  # Set true for builds targeting Android appliances.
+  is_android_appliance = false
+
   # Set true if the platform works as a single volume device, mapping all
   # volume streams to a single one.
   is_single_volume = false