Add metrics constant for data plan usage screen.

Change-Id: Ia5d13e4c3a8783e5c920791e7c2d6ef9b511b0bb
Note: This change is required for Data balance UI.
Bug: 62349208
Test: mmma frameworks/base/proto/
(cherry picked from commit 998f97f41d1d2c779202155e9c55361919eff365)
1 file changed