[statsd] Make executor thread a class member of MultiConditionTrigger

executorThread references class members after detaching. Making
executorThread as class member and joining in MultiConditionTrigger
destructor.

Ignore-AOSP-First: Security bugs merged into internal branch first
Test: atest statsd_test
Bug: 292160348
Flag: NONE mainline module bug fix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:30775d2bb2e2070bc49def70f97af4724342c4c2)
Merged-In: I7036eb3d506e8ca88e4a5faa6275dc4cba8020ee
Change-Id: I7036eb3d506e8ca88e4a5faa6275dc4cba8020ee
5 files changed