Retrieve shared lib output from custom provider

Use of the native CcSharedLibraryInfo results in using a superfluous
symlink for shared libraries in mixed builds. The new provider
propagates an unwrapped output file (real file, not a symlink).

Test: With related rules changes, manually verified that installed
mixed-build libc was not a symlink. (cf_x86_64, inspected system.img
with `m droid`)

Change-Id: I2a59b348fbfc2a381c229d66c0098e87674a9ddc