Inline some of the files into the tests

Modify LayoutPullParser construction to make clear what the different
constructors do and create a new one to build a parser just from a
String. By using this constructor, the CL inlines some of the smaller
files into the test for easier reading and maintenance.

Test: Modifies existing tests
Change-Id: Ieb20dcad4f62dc09252d01841bff37cf2c3c4a10