Set cameraserver optional.

Cameraserver is installed in handheld_system.mk, not
in TV and auto. Hence it is manually marked as optional.

Also update README to reflect what "optional" means.

Test: build cf_x86_auto
Bug: 37226359
Change-Id: Ib5e91c97256f9358074e68a67c535e25dec57409
diff --git a/vintfdata/frozen/5.xml b/vintfdata/frozen/5.xml
index 9c52d07..525829d 100644
--- a/vintfdata/frozen/5.xml
+++ b/vintfdata/frozen/5.xml
@@ -1,5 +1,9 @@
 <compatibility-matrix version="3.0" type="device">
-    <hal format="hidl" optional="false">
+    <!--
+         cameraserver is installed for all phones and tablets, but not
+         auto or TV.
+    -->
+    <hal format="hidl" optional="true">
         <name>android.frameworks.cameraservice.service</name>
         <version>2.1</version>
         <interface>