hiddenapi: Introduce greylist-max-p

Add a new hiddenapi::ApiList constant for a greylist only accessible to
apps targeting SDK version < Q.

The patch also removes SdkVersion::kP_MR1 which is invalid.

Test: m, phone boots
Test: m test-art
Test: atest art/tools/class2greylist/test
Change-Id: I48f3d1a4703166168fd8e0d1c3337aa2957b66ce