bp2build: handle system_shared_libs

- If no system_shared_libs is specified, bp2build writes no attribute
value. In this case, the bazel library macros determine the correct
default behavior.
- If any system_shared_libs is specified for any variant, then bp2build
writes the value verbatim. This includes if an empty list is specified,
as this should override defaulting behavior.

Note this defaulting behavior is incomplete and will be incorrect in
corner cases. For example, if, in an Android.bp, system_shared_libs is
specified for os.linux_bionic but not for os.android, then the bazel
default for os.android will be incorrect. However, there are no current
modules in AOSP which fit this case.

As a related fix, supports static struct for cc_library_static.

Also, removes some elements from the bp2build denylist.

Test: mixed_droid CI
Change-Id: Iee5feeaaf05e8e7209c7a90c913173832ad7bf91