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