Add install_symlink soong module type

This can be used to install symlinks to arbitrary locations/targets
on the device. Used to replace a make-built symlink.

Bug: 205632228
Test: built and ran the emulator observed the /system/bin/hwservicemanager symlink is still there
Change-Id: I6df922c8d919e6d56fa79702815a89c98f4d65ed

etc/prebuilt_etc_test.go

@@ -83,7 +83,7 @@ func TestPrebuiltEtcVariants(t *testing.T) {
 
 	baz_variants := result.ModuleVariantsForTests("baz.conf")
 	if len(baz_variants) != 1 {
-		t.Errorf("expected 1, got %#v", bar_variants)
+		t.Errorf("expected 1, got %#v", baz_variants)
 	}
 }