Rate limit BatteryStats' ModemActivityInfo querying

Frequent mobile radio power state changes results in frequent
ModemActivityInfo queries. To limit the unnecessary churn, a Modem
update due to mobile radio power state change will only occur some
amount of time after the previous update. Modem updates due to other
signals (plug/unplug events, battery percent change, etc) will be
unaffected.

Bug: 239715259
Test: atest BatteryStatsNoteTest
Change-Id: Ie0af28dbc2ad4836ef6437c0656dc20eec2285ea