Automatically add bootclasspath_fragment contents to sdk

Previously, both a bootclasspath_fragment and its contents had to be
explicitly added to the sdk. This change means that adding a
bootclasspath_fragment to and sdk will automatically add its contents
as if they were added using java_boot_libs.

Bug: 177892522
Test: m nothing
Change-Id: I8f7e70649f272c9a109d4606571a2d12c44b7904