Generate empty classpaths.proto for bootclasspath_fragment.go.

- Adds all required details for bootclasspath_fragment to implement
  classpath_fragment.
- Keeps the actual boot jars in platform-bootclasspath to begin with.
- Makes sure to put the file in apex/etc/classpath on device. Note that
  for platform versions of classpath fragment AndroidMkEntries perform
  the installation, while for APEXes it must be plumbed via apex.go.

Bug: 180105615
Test: m && launch_cvd; atest CtsClasspathsTestCases
Change-Id: I6101ebdf5b8bcbe95c0b7ce21f3f67a2685aef50