Heads up master switch (2/2)

Add an short explanation abaout how to enable Heads Up per app for those who don't like to read or serch in changelogs

Change-Id: I2f00ad71e564bc15c3904d295cfdccc4a4673996

Conflicts:
	res/values/cr_strings.xml
	res/xml/heads_up_settings.xml

Conflicts:
	res/values/cr_strings.xml
	res/xml/heads_up_settings.xml

Conflicts:
	res/values/simpleaosp_strings.xml
	src/com/android/settings/simpleaosp/HeadsUpSettings.java
Headsup:Actionbar Switch

Conflicts:
	res/values/simpleaosp_strings.xml
	src/com/android/settings/simpleaosp/HeadsUpSettings.java
Headsup tile (2/2)
Revert: Show headsup at bottom [2/2]