Use new naming scheme for TARGET_PLATFORM_VERSION

Replace the current naming scheme for TARGET_PLATFORM_VERSION
(O, ODR1, OMR1, P) with one that matches the user-visible
branch version (OPR1, OPD1, OPM1, PPR1).

Temporarily map P -> PPR1 to decouple this change from updating
the build servers.

Bug: 34972208
Test: m -j TARGET_PLATFORM_VERSION=PPR1
Test: m -j TARGET_PLATFORM_VERSION=P
Change-Id: Ib00b13abe79a744e344b88bb1d800524ab09d585