Revert "Revert "Add bootclasspath_fragments to platform-bootclas..."

Revert submission 14717811-revert-populate-platform-bootclasspath

Reason for revert: retry with a fix
Reverted Changes:
Ib58cd0211:Revert "Add bootclasspath_fragments to platform-bo...
I13b622d6c:Partial Revert "Populate individual classpath_frag...

Bug: 180105615
Test: atest sdkextensions_e2e_tests

Change-Id: Ic391fa4035f868b1ef92f5b275dad0ac5b1ab462