Make IdentityCredentialStore.getInstance() return null if credstore is not installed.

Having this method return null is the expected and documented behavior
when either the IC HAL or credstore isn't available.

Test: atest android.security.identity.cts (with credstore not running)
Bug: 148495024
Change-Id: Ifa17c58a84057499b1aeb8404959d5c0badfe52a
1 file changed