Set PLATFORM_SYSTEMSDK_MIN_VERSION := 28

In the temporary P-Finalization state, where PLATFORM_SDK_VERSION has
been incremented to 28 but PLATFORM_VERSION_CODENAME.PPR1 := P instead
of REL, PLATFORM_SYSTEMSDK_VERSIONS is still only P.  Set
PLATFORM_SYSTEMSDK_MIN_VERSION := 28 so that PLATFORM_SYSTEMSDK_VERSIONS
is 28 P.  Once PLATFORM_VERSION_CODENAME.PPR1 := REL it will become
just 28.

Bug: 78023556
Test: print PLATFORM_SYSTEMSDK_VERSIONS
Change-Id: I72cbef7bb7c84e6fd6fe4602021bb72d759d1b5e
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 698e221..8a1602e 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -210,7 +210,7 @@
   # However, currently in P, we only support the single latest version since there
   # is no old system SDK versions. Therefore, this is set to empty for now. This
   # should later (in post P) be set to a number, like 28.
-  PLATFORM_SYSTEMSDK_MIN_VERSION :=
+  PLATFORM_SYSTEMSDK_MIN_VERSION := 28
 endif
 
 # This is the list of system SDK versions that the current platform supports.