ANDROID: Make incfs selftests pass

Fixed incfs_test build errors
Fixed Kconfig errors
Readded .gitignore

Test: With just enabling CONFIG_INCREMENTAL_FS, both defconfig and
cuttlefish_defconfig build and incfs_test runs and passes

Bug: 133435829
Change-Id: Id3247ffcc63a095f66dcedf554017a06c5a9ce4a
Signed-off-by: Paul Lawrence <paullawrence@google.com>