Enable goma in soong

When the UseGoma flag is set, put all rules except the C compilation
rule in an externally defined local_pool, which will have been created
by kati.  The gomacc wrapper will already be in the CC_WRAPPER
environment variable.

Bug: 31142427
Change-Id: I699d4edff2e302eee398dad8692ceb14721a628c

android/variable.go

@@ -101,6 +101,7 @@ type productVariables struct {
 	Cpusets                    *bool `json:",omitempty"`
 	Schedboost                 *bool `json:",omitempty"`
 	Binder32bit                *bool `json:",omitempty"`
+	UseGoma                    *bool `json:",omitempty"`
 
 	DevicePrefer32BitExecutables *bool `json:",omitempty"`
 	HostPrefer32BitExecutables   *bool `json:",omitempty"`