Add libasound.so and conf files to prebuilts

These don't really belong with the vendor blobs -- they're built from
source (and in fact, the copies being committed are built directly from
a CM 10.1 source tree -- they're prebuilt for now to avoid having to
re-add external/alsa-lib to the CyanogenMod project).

Change-Id: Idb36a3805f119f5bbe356d6957f7b62f73be0390
proprietary-files.txt: remove libasound.so and other audio cruft

This stuff is either unnecessary or now residing in device/bn/encore.

Change-Id: Ia66795db44582d8c2708ebf7fac6fbb620b99817