Initialize two fields in constructor

mIsIndividualMic was potentially being read uninitialized in
onSensorPrivacyChanged().  In theory, mUserId isn't being read
uninitialized with the current logic, but we choose to
future-proof by initializing it to a known value.

Bug: 136069189
Test: TreeHugger
Change-Id: I60c76bfea0b224d9789ade2f2bdf35ffb8c1e361