Fix memory leak in TimeBase.mObservers list.

Leaks happen in following places:
1. Not all timer/counter in Uid class are detached from TimeBase when
the Uid object is destructed.
2. When Uid object is removed from mUidStats list in
onUserRemovedLocked, removeUidStatsLocked, readLocked, All
timer/counter in Uid class are not detached from TimeBase.
3. When timer/counter in Uid class is reassigned, the previous
timer/counter object is not detached.

Performance improvement:
The mObservers list in TimeBase object of BatteryStatsImpl can have
size up like 20k, the list type is ArrayList, remove() method on
ArrayList is very slow. For long mObservers list, we change to use
HashSet as container. The mObservers list in TimeBase object of Uid
is short, it continues to use ArrayList as container.

Fix: 80443940
Test: "adb shell cmd battery unplug" and "adb shell cmd battery set ac 1",
observe memory usage from Android Monitor.

Change-Id: I2001ca390a0a86a32082e1012dde4b831f1e7d04