Make getRootScope const

Add a getMutableRootScope method with original functionality.
getRootScope should not provide a pointer since callers should not be
modifying the scope directly. This changes all but one of the callers.

Bug: N/A
Test: ./test/run_all_host_tests.sh && ./test/run_all_device_tests.sh
Change-Id: I0b6ecad3322bb98fd7f1950ee25312c3a8c2faad
diff --git a/hidl-gen_l.ll b/hidl-gen_l.ll
index 835d8f6..16acc7f 100644
--- a/hidl-gen_l.ll
+++ b/hidl-gen_l.ll
@@ -198,7 +198,7 @@
 
     yyset_in(file.get(), scanner);
 
-    Scope* scopeStack = ast->getRootScope();
+    Scope* scopeStack = ast->getMutableRootScope();
     int res = yy::parser(scanner, ast, &scopeStack).parse();
 
     yylex_destroy(scanner);