indexed priority queue for AnomalyMonitor

Created a new class, indexed_priority_queue, which is a priority queue
that allows the removal of elements (other than just the top element).
This is required for AnomalyMonitor, which will henceforth use it.

Some tests for this new class are included.

I have only implemented the methods in indexed_priority_queue that are
currently needed. For example, pop_top() has not been written as
AnomalyMonitor does not need it.

Test: adb shell data/nativetest64/statsd_test/statsd_test
Change-Id: I82fe220ee5a879189b0cfa03b551c829cfdd05f0