Explicitly tag max-target-o APIs as "lo-prio".

This allows the presubmit checker to explicitly allow certain changes to
these APIs, rather than special casing tha max-target-o list. This in turn
will allow other APIs to be tagged as lo-prio in future to get the same
treatment.

Test: m out/soong/hiddenapi/hiddenapi-flags.csv
Bug: 172993934
Change-Id: Ic7cc0243b2b018c96d420a3f266c4b3a6958c32f