Show forwarded number in call screen

This change adds functionality to get forwarded number of current call.
This allows call-in-progress screen to show both forwarded number and
incoming callers number.

Test: manual - Confirmed that it stores forwarded number in
EXTRA_LAST_FORWARDED_NUMBER.
Test: auto - Passed FrameworksTelephonyTests
Bug: 29063067

Change-Id: I37a3196254c596e65bb9ab86b6d2e7b01273e323