AAPT: Handle gaps in resources when building splits

Due to public ID assignment, gaps in resource type and entry
IDs can occur. This can lead to null pointer dereferencing
if not careful. This happened in ResourceTable::getDensityVaryingResources()
which is called when building Splits.

Bug:30879690
Change-Id: I588e4dcd2e042fccfcb2e87967b5cbd0d23b4497