keyguard: add showCustomIntentOnKeyguard method

Extracted from here --> https://github.com/scotthartbti/android_frameworks_base/commit/89d203acd1ed22003e6fdb8211eee9d6ee92a7ad#diff-1

this is need for some slim features

PS2: fix derp

Change-Id: I476b94942b67843b0caab907bcee8f591740615e