AIDEGen: Add .gitignore for artifacts that aidegen created.

1. If .gitignore doesn't exist in target project folder, create it as a symbolic
link file of gitignore_template file. After putting gitignore_template in
aidegen git, Aidegen team can maintain it. If .gitignore exists, skip the
symbolic link creation. Because devs can maintain their .gitignore manually.
2. Remove non used constant and parentheses.

Bug: 133637941
Test: 1. $ m aidegen; aidegen-dev Settings -n.
      2. Check if .gitignore file is created.
      3. Change directory to /packages/apps/Settings, $ git status should not
         show as following:
         Untracked files:
         	 .idea/
	         Settings.iml
	         dependencies-Settings.iml

Change-Id: Ib24dad9945a192483c922578c471f7237422e461