Add APIs to getDefaultDialerPackage and getSystemDialerPackage

getDefaultDialerPackage is a public API and will be used by
apps to determine if they are the default dialer.

getSystemDialerPackage is a hidden API and is only used by
system components.

Also add new public string constants to allow apps to show
a dialog requesting that the user change the current default
dialer.

Bug: 20304458
Change-Id: I121d36741474c1fe171c372fb766a6c128228c9c