[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:4196da2ec586a631fc9011c7615f80f4f197caf3)
Merged-In: I7036eb3d506e8ca88e4a5faa6275dc4cba8020ee
Change-Id: I7036eb3d506e8ca88e4a5faa6275dc4cba8020ee
4 files changed