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/config.go

@@ -369,6 +369,10 @@ func (c *config) Android64() bool {
 	return false
 }
 
+func (c *config) UseGoma() bool {
+	return Bool(c.ProductVariables.UseGoma)
+}
+
 func (c *config) LibartImgHostBaseAddress() string {
 	return "0x60000000"
 }