java_sdk_library: Correct stubs source generation classpath

The stubs source should be generated using the same classpath that is
used to compile the implementation library, i.e. sdk_version,
system_modules and libs/static_libs.

Previously, the sdk_version used to generate the stubs was different.
It provided access to a wider API surface than was necessary. This
change fixes that.

Test: m checkapi
Bug: 155164730
Merged-In: I7bc54dc4ad12a7d6c7503111d1322ee35c56d713
Change-Id: I7bc54dc4ad12a7d6c7503111d1322ee35c56d713
(cherry picked from commit 7b78b4d4)