Propagate empty vs unspecified system_shared_libs correctly.

Necessary to get correct prebuilts for many Bionic libs.

Cleaned up numerious "system_shared_libs: []" from test fixtures, since
they otherwise would need correction in the expected results, and it is
better to have a single test focused on testing system_shared_libs
propagation.

Test: m nothing
Bug: 152255951
Merged-In: If2e8a5296223e6281d833312660e8e9e4cd184c0
Change-Id: If2e8a5296223e6281d833312660e8e9e4cd184c0
(cherry picked from commit 10566a03)