Add C++ Host support on sysprop_library

With Host_supported: true, C++ part of sysprop_library will create host
variant which can be used from host modules. As there are no native
system property support on host, libbase functions will be used instead.
Adding support on host will help reduce code complexity of other
host_supported modules.

Bug: 147708854
Test: m, sysprop_test, manually test host binary
Change-Id: I850d91fea298ef1a0c16c6a7a9ec1aca5cf37e69

cc/testing.go

@@ -188,6 +188,7 @@ func GatherRequiredDepsForTest(os android.OsType) string {
 			stl: "none",
 			vendor_available: true,
 			recovery_available: true,
+			host_supported: true,
 		}
 		cc_library {
 			name: "libc++",
@@ -197,6 +198,7 @@ func GatherRequiredDepsForTest(os android.OsType) string {
 			stl: "none",
 			vendor_available: true,
 			recovery_available: true,
+			host_supported: true,
 			vndk: {
 				enabled: true,
 				support_system_process: true,