Adding @SystemApi annotation to Bluetooth energy stats.

Added @SystemApi annotation to BluetoothActivityEnergyInfo
and UidTraffic getters for mainline compatibility. Replaced
raw array by List. Deleted unused constructors.

Tag: #feature
Bug: 200201373
Test: Manual
Change-Id: Id93215f48bf08d31ac269af9e064cf598f50b6d3