Change classpath for droiddoc

When the module type is SdkLibraryDependency, the classpath has been
gotten from headerJar
Becuase droiddoc uses src files when it builds, we should change to use
ImplementationJars instead of headerJar.

Bug: 77577799
Test: make -j

Change-Id: I1a072be69d7edff5636ea80af700be7796c3b0fc