Make apex_available respect AllowMissingDependencies

When SOONG_ALLOW_MISSING_DEPENDENCIES is defined in the build, don't
fail if an apex_available entry is not available. This is for smaller
Android manifests that may not include all APEX projects (e.g. master-art).

Bug: 142300164
Test: able to build from master-art manifest.
Change-Id: Ice1274f7dd143c4b1bf756e2cde93e217266affa

android/apex.go

@@ -155,7 +155,7 @@ func (m *ApexModuleBase) checkApexAvailableProperty(mctx BaseModuleContext) {
 		if n == availableToPlatform || n == availableToAnyApex {
 			continue
 		}
-		if !mctx.OtherModuleExists(n) {
+		if !mctx.OtherModuleExists(n) && !mctx.Config().AllowMissingDependencies() {
 			mctx.PropertyErrorf("apex_available", "%q is not a valid module name", n)
 		}
 	}