SecuritySettings: Fix KeyStore related NPE's

Both "Show password" and "Credential storage" options depend on having
a KeyStore. However, KeyStore assumes that there's an
android.security.keystore service around. Let's validate if it indeed
exists and not show those two sections if it doesn't, in order to
avoid breakage.

Issue: BUGDUMP-7995106

Change-Id: I30b74057aae42e72387087d1c1d847dd8524f87f