Only set TARGET_PLATFORM version in lunch when explicitly requested

Setting TARGET_PLATFORM_VERSION to DEFAULT_PLATFORM_VERSION during
"lunch sailfish-userdebug" causes unnecessary pain when
DEFAULT_PLATFORM_VERSION becomes invalid after branching.  Only
set TARGET_PLATFORM_VERSION if it was explicitly requested with
lunch sailfish-userdebug-OPR1.

Test: build/make/tests/envsetup_tests.sh
Bug: 34972208
Bug: 37208937
Change-Id: I40ba4617e73803c5bec1a8d317382f70fb3ec3a2
Merged-In: I40ba4617e73803c5bec1a8d317382f70fb3ec3a2