Explicitly include libgui as a shared library in libgui_test.

Currently, libgui is compiled statically into the test binary by virtue
of `src` attribute of `libgui-defaults`, and also loaded as a shared
library by transitive dependency.  This leads to double-delete and
use-after-delete issues in the test on global mutexes defined by
android::Singleton<>.

Flag: EXEMPT refactor
Test: presubmit
Bug: 399711310
Change-Id: I31408237f345632bfd8f2d2ac880dfac53e75ae4