Always use TypedArray to obtain manifest values

In some paths, the "isolatedSplits" attribute was read directly from
the XML file. In others, it was read using a TypedArray [ie. it used
the application's resource table].

It's possible we should never allow some manifest values to be set
via resource table. This change does not attempt to resolve that.
Instead, it merely ensures consistency to how manifest values are
retrieved.

Bug: 150184652
Test: atest PackageParserTest
Change-Id: Ib7c6a5ae081959e2d3aba4abdf60dd52db05991a