Cleanup dvr_api.h to make it closer to the exported SDK version

Also fixed include guards on dvr_display_manager.h to avoid
collision with display_manager_client.h.

build_sdk.py has been moved to vendor/google_daydream/build.

Bug: b/38379392
Test: Built libdvr, surfaceflinger, ran libdvr/tests
Change-Id: I2807be6d2fd5c2814e86e5d8fae8de181397e4a6