Prevent PIE recent button from changing to clear all button on lockscreen

Follow up to previous pull request/commit 
See discussion here - https://github.com/ParanoidAndroid/android_frameworks_base/pull/40

When on the lockscreen (protected or unprotected), the clear all button is no longer accessible. This applies to "full" PIE and "bare" PIE. Once the screen is unlocked, the button functions as usual (recent switches to clear all when one or more apps are present in the recent apps view)