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
Change-Id: If2e8a5296223e6281d833312660e8e9e4cd184c0

cc/cc.go

@@ -378,7 +378,7 @@ type linker interface {
 
 type specifiedDeps struct {
 	sharedLibs       []string
-	systemSharedLibs []string
+	systemSharedLibs []string // Note nil and [] are semantically distinct.
 }
 
 type installer interface {