Call ActivityManagerService.enterSafeMode

This fixes the accidental removal (change ID
If2e6afe21f6efcb141f3a4428ff9154b68f08a1d)
of a call to ActivityManagerService.enterSafeMode, ensuring that
when the rest of the device is in safe mode, ActivityManagerService
and PackageManagerService are made aware of it and properly filter
app launches.

Fixes: 137052912
Test: manual; safe mode correctly blocks 3p apps
Change-Id: Ie2caf6d2fd74b241927a245393fb31658201962f
Merged-In: Ie2caf6d2fd74b241927a245393fb31658201962f