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: I2c4cfe587ad4b3d459c3610bbf3928fb2f8ccea7
Merged-In: I2c4cfe587ad4b3d459c3610bbf3928fb2f8ccea7