Move the initialization of Call/SMS/MMS content observers out of the DataManager constructor

This change is to speed up the system boot time. The people service
start time will be reduced from ~1.5 ms to ~0.5 ms.

Bug: 149245782
Test: Manual run with System.nanoTime() logged
Change-Id: I9114b3c9e3637b2498618220665cd98df1d1f994