Remove the Build.VERSION_CODES.P0

The Build.VERSION_CODES.P0 was the stub for a potential new API level
after P. Now it's Q thus the usage for P0 field is not valid any more.
This CL also fixed the android.os.cts.BuildTest test failure for
cf_x86_phone-userdebug

Bug: 131601118
Test: presubmit test on ag/7204147
Change-Id: Iee474e95a35e051e6a3f4f96da5d82387d33d013