Convert libaaudio to Android.bp

See build/soong/README.md for more information about Soong.

I didn't translate the examples, since they appear to be NDK build
files, not platform build files.

Test: mmma frameworks/av/media/libaaudio
Change-Id: Icf228c701afe37dd3c7f00ea9e6fb740b0d450f1
Merged-In: Icf228c701afe37dd3c7f00ea9e6fb740b0d450f1
Exempt-From-Owner-Approval: simple backport
(cherry picked from commit 0230b1ac)