Just a small input to this point:

On Android 4.4 the new host card emulation (HCE) feature (aka: the phone emulates a ISO-DEP Smartcard and processes ISO7816-4 APDU commands like a Smartcard would do) only works when the display is on, but even when the screen is locked (can be changed with "android:requireDeviceUnlock" in Manifest). See here for detailled specification: http://developer.android.com/guide/topics/connectivity/nfc/hce.html

Using the HCE API on Android 4.4 also has the beauty that any app that registers itself for HCE and sets its category to CATEGORY_PAYMENT in the Manifest automatically shows up in Adroid's system settings under "Tap & Pay" (where a user would expect payment applications).