Default apex_available to //apex_available:platform

This means everything that goes into apexes need to be
explicitly labeled to be available for apex.

Whitelist the current offenders. This list should be
trimmed down.

Bug: 147364041
Test: m
Test: multiproduct_kati -only-soong
Exempt-From-Owner-Approval: cherry-pick from internal
Change-Id: I837299c6a15d46f8a5ba544b613776b1cc27d7b8
Merged-In: I837299c6a15d46f8a5ba544b613776b1cc27d7b8
(cherry picked from commit 93488cbb10d4882845abb732f8e53714f0982031)

sdk/java_sdk_test.go

@@ -73,6 +73,10 @@ func TestBasicSdkWithJavaLibrary(t *testing.T) {
 			sdk_version: "none",
 			compile_dex: true,
 			host_supported: true,
+			apex_available: [
+				"myapex",
+				"myapex2",
+			],
 		}
 
 		apex {