Split buckets on boot complete

Also clean up a bit of code on splitting on app upgrades

Piggy-backed off the app upgrade tests, adding parameterized tests to
also test boot complete event.

Refactored some value metric test code to increase code reuse and
assertions.

Fixed a broken value metric test that had assertions commented out.

Refactored NamedLatch into MultiConditionTrigger to avoid creating a
thread before necessary.

Test: atest statsd_test
Test: push a simple test config, reboot, wait, get data. Made sure
the bucket was split
Bug: 144099206
Bug: 154511974

Change-Id: I73858b5db08e8cda762bd8091b30da8738d1fd88