atv-sdk: use google as brand instead of generic_x86

Current ro.build.fingerprint on oc-release has 92 chars as shown below:
generic_x86/sdk_google_atv_x86/generic_x86:8.0.0/OPR1.170516.001/4012038:userdebug/test-keys
we can shorten it by 5 chars with google brand, similar to other google sdk images
such as sdk_gphone_x86 target.

following is the new fingerprint 87 chars after replacement
google/sdk_google_atv_x86/generic_x86:8.0.0/OPR1.170516.001/4012038:userdebug/test-keys

BUG: 38352731
Change-Id: I938341ac3c11ebe6d966b2cdb14bb2b289ef7ef7