Call ActivityManagerService.enterSafeMode

This fixes the accidental removal (change ID
Ibe849f56f5fe8af1415dc6c85b484d0edca518ec)
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: 129781631
Test: manual; safe mode correctly blocks 3p apps
Change-Id: Ie2caf6d2fd74b241927a245393fb31658201962f