Update .gitignore to better deal with .idea files.
Update .gitignore to better deal with .idea files.

We want to ignore all the files except the ones already checked into
git. This changes to do that instead of building a blacklist as Intellij
might add more new files that we do not have listed yet.

Test: None
Change-Id: Ia145be246947b6e22f6db0968e7748e5bd08a33f

aosp-mirror/platform_frameworks_support,aosp-mirror/platform_frameworks_support,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,AndroidX/androidx