libaom: use chromium config script

Modify chromium/src/third_party/libaom/cmake_update.sh and use the file
list it generates.

Keep update_libaom.sh as close to update_libvpx.sh as possible.

Use config options from chromium for security and performance.

Rename files to avoid object file collision. Will be upstreamed.

Disable RTCD because Android mandates NEON and SSSE3. Enables stripping
unused functions. If SSE4 or higher is desired, this will need to be
revisited.

Test: none

BUG=122391143

Change-Id: Ia397022c0ef6f93901daa95da84de6a6f4702c7e