Make ModemActivityInfo isValid system

Make ModemActivityInfo.isValid system because
 getIdleTimeMillis(), getSleepTimeMillis(), and getReceiveTimeMillis()
 are system.

Bug: 146436760
Test: build
Change-Id: Ifc1a1853fe0b7ca143ef95fe3a1abe70e2dc70d4
Merged-In: Ifc1a1853fe0b7ca143ef95fe3a1abe70e2dc70d4
(cherry picked from commit 3349553a99dad355886bc97e655e1f84d35011d8)
2 files changed