Proto enums for DeviceIdleMode

Created an enums.proto that contains the device idle modes,
which is referenced by batterystats.

Note that, currently, batterystats is the origin of these
constants in the java code, and I have kept that as is.
Thus, batterystats references the enum.
Nevertheless, because the actual control of device idle
mode is DeviceIdleController.java, where these constants
will likely one day be moved, I have put the constants
in server/enums.proto (since DeviceIdleController is
in server, not os like BatteryStats).

Bug: 69478930
Test: cts-tradefed run cts-dev -m CtsStatsdHostTestCases -t android.cts.statsd.HostAtomTests#testDeviceIdleModeStateChangedAtom
Change-Id: I6e66801ae8256aab423067f9a4b852a194564a8d