Move DVR platform C API into libdvr

Introduce a new static library build with Android.mk. The reasoning
behind this is:
1/ This library is going to consolidate all C headers and implementation
of the DVR platform library.
2/ No other internal system components need to depend on this library.
3/ This library is build with Android.mk so that it has depend on other
system components (such as libandroid_runtime) without worrying about
that Android.bp-based libs cannot depent and Androib.mk-based libraries.
4/ This library defines dvr_api.h header as the official definition of
DVR platform library headers. The header is moved from
'vendor/unbundled_google/packages/PrebuiltGoogleVr', which is not an
ideal place to hold the platform library anyway.
5/ If we ever decide to move these C wrappers out of frameworks/native,
this makes it easier to do so by bundling things now.

TODO: C APIs from libvrsensor and libvirtualtouchpadclient are not moved
into this bundle as some functions are still used by internal system
components. Ideally, these will be moved either to Google3 or vendor projects.

Test: Built and flash
Bug: 36563654
Change-Id: Ie5ce89faa70f86ac934a77d8f7d4fca7e94112ee