Adds support for 'ignored-on-host'

Adds a filter mechanism that can exclude property values from being
included in the common value extraction. That is needed to prevent the
snapshot mechanism from generating invalid output for properties that
are ignored on host (and have their values cleared) and which are not
tagged with `android:"arch_variant"`.

Changes:
* Updates the documentation of SdkMemberType to explain what effect
  the 'ignored-on-host' tag has.
* Adds some tests for this new mechanism.

Bug: 155628860
Test: m nothing
Change-Id: I7ebd333079619dba546bc8c4911d567e0287b676
diff --git a/sdk/sdk_test.go b/sdk/sdk_test.go
index 898ecea..ae1a492 100644
--- a/sdk/sdk_test.go
+++ b/sdk/sdk_test.go
@@ -289,9 +289,12 @@
 	}
 
 	extractor := newCommonValueExtractor(common)
-	extractor.extractCommonProperties(common, structs)
 
 	h := TestHelper{t}
+
+	err := extractor.extractCommonProperties(common, structs)
+	h.AssertDeepEquals("unexpected error", nil, err)
+
 	h.AssertDeepEquals("common properties not correct",
 		&testPropertiesStruct{
 			name:        "common",