Fix sdk_version: "system_current" when Platform_sdk_final=true

When PLATFORM_VERSION_CODENAME is set to REL Platform_sdk_final
becomes true, which causes the return value of sdkVersionToNumber
for "system_current" to a real version number instead of
FutureApiLevel.  This enables the check against
PlatformSystemSdkVersions, which doesn't contain "current".  Use
the numeric value instead.

Fixes: 129786845
Test: sdk_test.go
Change-Id: If7cf211cc01c5fbf3e3ece3c3f604718a13d5a9b