rust: Add a default cfg indicating an AOSP build

This adds a cfg flag `android_platform` which can be used
to indicate there's some conditional compilation need specific to
AOSP.

Test: New flag is emitted on builds
Change-Id: I833a7d68db2dad7ab2d32a5eb51d764f0d66b849

rust/config/global.go

@@ -53,6 +53,9 @@ var (
 		"--color=always",
 		"-Z dylib-lto",
 		"-Z link-native-libraries=no",
+
+		// cfg flag to indicate that we are building in AOSP with Soong
+		"--cfg soong",
 	}
 
 	LinuxHostGlobalLinkFlags = []string{