rust: Emit android vndk cfg flag.

Pass an "android_vndk" cfg flag that indicates this code
targets the vndk. This can be useful in instances where code
might need to behave differently.

This also includes a fix to make sure our vendor Soong tests
are correctly configured.

Bug: 179907868
Test: Soong tests pass.
Test: Example module emits new cfg flags.
Change-Id: I01cdf91f6f9d42cd8a759266d5170479664bf4bc

rust/rust_test.go

@@ -64,6 +64,14 @@ func testRust(t *testing.T, bp string) *android.TestContext {
 	return tctx.parse(t)
 }
 
+func testRustVndk(t *testing.T, bp string) *android.TestContext {
+	tctx := newTestRustCtx(t, bp)
+	tctx.useMockedFs()
+	tctx.generateConfig()
+	tctx.setVndk(t)
+	return tctx.parse(t)
+}
+
 // testRustCov returns a TestContext in which a basic environment has been
 // setup. This environment explicitly enables coverage.
 func testRustCov(t *testing.T, bp string) *android.TestContext {
@@ -140,6 +148,15 @@ func (tctx *testRustCtx) enableCoverage(t *testing.T) {
 	tctx.config.TestProductVariables.NativeCoveragePaths = []string{"*"}
 }
 
+func (tctx *testRustCtx) setVndk(t *testing.T) {
+	if tctx.config == nil {
+		t.Fatalf("tctx.config not been generated yet. Please call generateConfig first.")
+	}
+	tctx.config.TestProductVariables.DeviceVndkVersion = StringPtr("current")
+	tctx.config.TestProductVariables.ProductVndkVersion = StringPtr("current")
+	tctx.config.TestProductVariables.Platform_vndk_version = StringPtr("VER")
+}
+
 // parse validates the configuration and parses the Blueprint file. It returns
 // a TestContext which can be used to retrieve the generated modules via
 // ModuleForTests.