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.

Bug: 20304458
Change-Id: Ifed855ccb8827cb2f62fd13f7c5afc307425ef29
1 file changed