Optimize out empty path components

filepath.Join("foo", "") returns a newly allocated copy of "foo",
while filepath.Join("foo") does not.  Strip out any empty path
components before calling filepath.Join.

Test: TestValidatePath
Change-Id: Ib47dbcd9d6463809acfe260dfd9af87ea280b4de

android/paths_test.go

@@ -36,6 +36,22 @@ var commonValidatePathTestCases = []strsTestCase{
 		in:  []string{""},
 		out: "",
 	},
+	{
+		in:  []string{"", ""},
+		out: "",
+	},
+	{
+		in:  []string{"a", ""},
+		out: "a",
+	},
+	{
+		in:  []string{"", "a"},
+		out: "a",
+	},
+	{
+		in:  []string{"", "a", ""},
+		out: "a",
+	},
 	{
 		in:  []string{"a/b"},
 		out: "a/b",