Fix race between SensorManager ctor and callback

Avoids potential invalid memory access if system server crashes during
initialization.

Bug: 132927376
Test: confirm sensors initialize
Change-Id: If7421c452b5893ab2567323d265503a1ce304482
Merged-In: If7421c452b5893ab2567323d265503a1ce304482
(cherry picked from commit 4b23ed10)