Fixed NPE in

com.android.packageinstaller.InstallStart#declaresAppOpPermission

Failing CTS tests in
android.packageinstaller.externalsources.cts.ExternalSourcesTest
reveal a NPE in InstallStart#declaresAppOpPermission. Turns out
android.content.pm.IPackageManager#getAppOpPermissionPackages can validly
return null, yet this code does not check for null before iterating.

Bug: 62272351

Test: Re-ran CTS test: run cts -m CtsExternalSourcesTestCases
-t android.packageinstaller.externalsources.cts.ExternalSourcesTest -l
DEBUG -s 01f2301c0f6ac226 -a armeabi-v7a
(cherry picked from commit 67f7aaef9646b5fcc6d1efcd10e623e9add33e7c)

Change-Id: I2637b4fd12f0d8487211f411bee850571d8bad9a