Add APIs to return max possible active phones.

This is first step towards reboot free single SIM to DSDS switch. As
getPhoneCount returns active phone count based on current configuration,
getMaxPhoneCount (subject to renaming) will return max possible active
phones(logical modems).

Bug: 141023026
Test: manual

Change-Id: Icc6e7da7c065eb3f72ec85a99ccc148639c574da
Merged-In: Icc6e7da7c065eb3f72ec85a99ccc148639c574da