Handle race conditions in SCS when statsd dies

This CL aims to fix two race conditions:

1. When statsd restarts after a crash, the ordering of sayHiToStatsd and
binderDied is not guaranteed. However, previously, we assumed that
binderDied would get called first and reset sStatsd to null. To solve,
we don't assume a function ordering and don't throw an error message in
sayHiToStatsd if sStatsd is not null.

2. When statsd was linked to death, the death recipient was not informed
about all broadcast receivers. Thus, the death recipient might have
known only a partial list of receivers when #binderDied was triggered. To
solve, we make sure that the death recipient knows about all receivers
before we link to death.

Test: atest statsd_test
Test: atest CtsStatsdHostTestCases
Bug: 154275510

Change-Id: I11be65ca2135cde200ab8ecb611a363d8f7c2eb6