Show current setting for Tap & Pay

In Settings > Apps & notifications > Advanced > Default apps > Tap & pay
the setting labeled "Use default" is a drop down preference but wasn't
showing the current value it's set to in its summary. This CL fixes
that.

Change-Id: I891289abad781c3eeafa2d7979373f0d53faafc6
Fixes: 67754372
Test: make -j64 RunSettingsRoboTests