Create bp2build converter for cc_shared_library

Factor out shareable code with cc_static_library

Remove `Linkstatic` field for shared cases,
because it's redundant with the rule

Bug: 198241907
Test: cc_library_shared_conversion_test.go
Test: mixed_{libc,droid}.sh
Change-Id: I5be3b66f812893bce41edb425fbd143ab22d9051