[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