add function to traversal the crates

The system_server needs to traversal Context.getCratesDir() to get all
of crates. But, system_server doesn't have the permission. system_server
needs installd to traverse Context.getCratesDir().

The depth of traversal in Context.getCratesDir is 1. installd returns the
list CrateMetadata. CrateMetadata contains the following information.
* crateName: it is the folder name

To add .gitigTo add .gitignore to collect the ignore file list.

Test: atest \
        CtsOsTestCases:android.os.storage.cts.CrateInfoTest \
        CtsOsTestCases:android.os.storage.cts.StorageManagerCratesTest \
        CtsOsTestCases:android.os.storage.cts.StorageStatsManagerTest

Bug: 141660526
Change-Id: Iea5eee4606e4ff437edef617a1f8db93e37f51c4