Lazily initialize the view package's default worker

[brandur]

See discussion in #1191 for impetus, but in general, it's not
particularly good practice to start up goroutines in package init
functions, with one reason being that it means that they're started for
projects that don't even make use of the package, and maybe just have it
as a transitive dependency.

So here, remove the view package's default worker in favor of a new
function that'll lazily initialize one when any package-level functions
are called that use the default worker. We use a sync.Once that has an
optimized short-circuit path in case the work's already been done, so
new overhead should be negligible, and registering/unregistering views
is probably not a particularly common operation anyway.

This change is backwards compatible, with use of the package's API
staying exactly the same. Test coverage seems to be pretty good, with a
function that previously reset the default worker between test cases
which I've modified to instead reset the default worker back to its
uninitialized state, and thereby verify that the test cases can
initialize it if necessary.

I thought about trying to deinitialize the default worker in case its
last view is unregistered, but decided against it because (1) it's more
invasive, (2) it's not likely to actually help anyone in practice as
views probably stay registered for a program's entire duration anwyay,
and (3) its interaction with the existing package-level Stop function
would take some thinking through.

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[brandur]

I noticed there seems to be a pre-existing intermittent test problem in the package which can be repro'ed on master with something like:

$ go test ./stats/view -run Test_Worker_MultiExport -count=20
--- FAIL: Test_Worker_MultiExport (0.00s)
    worker_test.go:217: Mismatched value (want 2, got 1) for &{[{a true}] [{2022-12-11 13:06:06.331908 -0800 PST m=+0.003086001 1}] 2022-12-11 13:06:06.331901 -0800 PST m=+0.003079585} in "/VF1"
    worker_test.go:222: Mismatched value (want 7.500000, got 2.000000) for &{[] [{2022-12-11 13:06:06.331908 -0800 PST m=+0.003086001 2}] 2022-12-11 13:06:06.331901 -0800 PST m=+0.003079585} in "/VF2"

I didn't take a stab at it though because I feel like someone else would be able to fix this a lot quicker.

[dashpole]

Fixing presubmit: #1288