Add bootclasspath_fragment.

Note that the contents of the bootclasspath_fragment are added as
dependencies to an apex, so there is no need to duplicate them in
java_libs.

Bug: 180105615
Test: atest CtsClasspathsTestCases
Change-Id: I1537d8a30c67995955af2d2d1bb2c11f01a24fe9
Merged-In: I1537d8a30c67995955af2d2d1bb2c11f01a24fe9