Add IsHdmiPlatform to AvSettings.

This provides the owner of AvSettings to identify if HDMI
specific behavior is available.

Bug: internal 146016184
Test: Verified on device and cast tryjobs.
Change-Id: I5eeec50e19e73c10a41205d26ff77ce15aa9daf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993654Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Chris Letnick <cletnick@google.com>
Cr-Commit-Position: refs/heads/master@{#730300}

chromecast/public/avsettings.h

@@ -246,6 +246,10 @@ class AvSettings {
   // This function should only be implemented on HDMI platforms.
   static CHROMECAST_EXPORT int GetHdmiLatencyUs() __attribute__((weak));
 
+  // Returns true if this is an HDMI platform.
+  // This function should only be implemented on HDMI platforms.
+  static CHROMECAST_EXPORT bool IsHdmiPlatform() __attribute__((weak));
+
   // Returns the type of volume control, i.e. MASTER_VOLUME, FIXED_VOLUME or
   // ATTENUATION_VOLUME. For example, normal TVs, devices of CEC audio
   // controls, and audio devices are master volume systems. The counter