Support complicated variable references

Bug: 226974242
Test: go test
Change-Id: Iaec16f5c498e7c75c9ee5d53d3499efadfba16bc

androidmk/parser/make_strings.go

@@ -234,10 +234,10 @@ func (ms *MakeString) splitNFunc(n int, splitFunc func(s string, n int) []string
 		if n != 0 {
 			split := splitFunc(s, n)
 			if n != -1 {
-				if len(split) > n {
+				if len(split) > n || len(split) == 0 {
 					panic("oops!")
 				} else {
-					n -= len(split)
+					n -= len(split) - 1
 				}
 			}
 			curMs.appendString(split[0])

androidmk/parser/make_strings_test.go

@@ -75,6 +75,16 @@ var splitNTestCases = []struct {
 			genMakeString(""),
 		},
 	},
+	{
+		// "x$(var1)y bar"
+		in:  genMakeString("x", "var1", "y bar"),
+		sep: " ",
+		n:   2,
+		expected: []*MakeString{
+			genMakeString("x", "var1", "y"),
+			genMakeString("bar"),
+		},
+	},
 }
 
 func TestMakeStringSplitN(t *testing.T) {