Update TelMan API for testing Line1NumberForDisplay.

CTS testing is being added for
TelephonyManager#setLine1NumberForDisplay. This requires making the
getter for alphaTag visible for testing with the @TestApi annotation.

Bug: 31963371
Test: API change. tests are unaffected.
Merged-In: I85887cf05f395f4ac4ca8dc6800e29f4fc9f8de4
Change-Id: I85887cf05f395f4ac4ca8dc6800e29f4fc9f8de4
(cherry picked from commit 710cce6d)