Ensure ActivityManagerInternal instance is non-null

Pulling up the initialization to onStart to avoid having earlier code
run into uses of mActivityManagerInternal.

Test: atest CtsAlarmManagerTestCases
atest FrameworksMockingServicesTests:AlarmManagerServiceTest

Fixes: 159651473
Change-Id: I0e06e2062603272eb22b534a81d7c2f0a5236a17