RRO for framework-res.apk is not force built against SystemSDK

When BOARD_SYSTEMSDK_VERSIONS is set, any apk in the vendor partition
is force built against SystemSDK (or Android SDK if LOCAL_SDK_VERSION is
set to current). However, there should be one exception to this: RRO for
framework-res.apk. This is because the RRO is currently overriding
resources that are not part of SystemSDK. Until SystemSDK is expanded to
cover resources that are eligible for overriding, let's allow the direct
dependency from RRO to framework-res.apk.

Bug: 35859726
Bug: 76398918
Test: m -j for 2018 Pixels
Change-Id: Ib9981b107d78691eef21f4525ebdac9866d5c50e