Avoid system crash because null AndroidPackage

mConnection.getPackage(), it may return nullable, the system should
not call buildHostsString(pkg) if pkg is null.

Bug: 318854490
Test: Cannot reproduce, just run atest StagedInstallTest
\#testInstallStagedApex_SameGrade

Change-Id: I3b0762f2a120e3fe460cc686e30ddffc57543205