Add all .idea directories to .gitignore

aidegen can be used to generate projects for specific packages within
frameworks/base, so it seems reasonable to ignore all of these
directories.

Bug: N/A
Test: git status
Change-Id: Ica235cafc008174162e64028420a9a494877ff92
diff --git a/.gitignore b/.gitignore
index d7aebc6..481c592 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,3 @@
-/.idea
+.idea
 *.iml
 *.sw*