Build VNDK with VNDK prebuilts without BOARD_VNDK_VERSION

Current VNDK prebuilts are disabled if BOARD_VNDK_VERSION is not set,
but VNDK prebuilts should be enabled even BOARD_VNDK_VERSION is not set
because VNDK APEXes should be available from VNDK deprecation. This
change removes some restrictions on VNDK prebuilts to enable those from
VNDK deprecation.

Bug: 316829758
Test: AOSP Cuttlefish build succeeded without BOARD_VNDK_VERSION
Change-Id: Id780811dab26f2125097c3efc5b2b4a59416b826

apex/apex_test.go

@@ -3768,13 +3768,6 @@ func TestVndkApexCurrent(t *testing.T) {
 				"lib64/libvndk.so",
 				"lib64/libvndksp.so"),
 		},
-		{
-			vndkVersion: "",
-			expectedFiles: append(commonFiles,
-				// Legacy VNDK APEX contains only VNDK-SP files (of core variant)
-				"lib/libvndksp.so",
-				"lib64/libvndksp.so"),
-		},
 	}
 	for _, tc := range testCases {
 		t.Run("VNDK.current with DeviceVndkVersion="+tc.vndkVersion, func(t *testing.T) {