Move SIP settings to phone account settings.

+ Moved "SIP accounts" and "Receive incoming calls" settings from
the SIP settings creen to the Phone account settings screen.
+ To facilitate this, I moved some helper functions which used to
be in SipSettings to SipUtil. Their logical innards are for the
most part unchanged.
- Removed "SIP settings" preference screen added in call settings.
- Removed "Receive incoming calls" from SipSettings activity.
- Deleted unused SIP settings layout.
+ Changed back button behavior in SIP settings activity so that
back takes it to phone account settings, rather than call feature
settings.
+ Made some necessary classes/functions in sip/ public. Ihab said
that these were this way because of some refactoring at some point,
and it should be fine to make them public to achieve the necessary
class accessibility to facilitate this CL.

Bug: 17487262
Change-Id: I8e24abf624cadf9afc7aeff28781802732fef07d
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 02eb9c8..72cd63e 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -149,6 +149,8 @@
     <string name="wifi_calling_do_not_use">Do not use Wi-Fi calling</string>
     <!-- Indication for option to not use a call assistant. -->
     <string name="wifi_calling_do_not_use_call_assistant">Do not use call assistant</string>
+    <!-- Indication for option to not use a call assistant. -->
+    <string name="wifi_calling_call_assistant_none">None</string>
     <!-- Label for selected Wi-Fi call manager -->
     <string name="wifi_calling_call_assistant">Call assistant</string>
     <!-- Label for switch setting to enable/disable the use of SIM call managers. -->