Fix crash when launching Activities in 'android' package

The LoadedApk for the 'android' package doesn't have a resource
directory set. This is fine since the AssetManager loads the
'android' package's resources by default.

This is a regression caused by https://googleplex-android-review.googlesource.com/#/c/525840/

Bug:17104110
Change-Id: I8a2158a75b4ff0e941837c11a2cb77a7aad09270