Initial Fuchsia support.

This change adds Fuchsia as a valid OS. Future changes
will add proper toolchain support.

Bug: 119831161
Test: Compile walleye. Confirm that adding `fuchsia` to the
os-specific declaration of an arbitrary target does not throw
a build error.
Change-Id: I64eb928afb7512f3fbe32abb313b4c3efe16b169

android/variable.go

@@ -243,6 +243,8 @@ type productVariables struct {
 
 	Product_is_iot *bool `json:",omitempty"`
 
+	Fuchsia *bool `json:",omitempty"`
+
 	DeviceKernelHeaders []string `json:",omitempty"`
 
 	ExtraVndkVersions []string `json:",omitempty"`