runas: grant access to seapp_contexts files

To be replaced by commit 1e149967
seapp_context: explicitly label all seapp context files

Test: build policy
Change-Id: I8d30bd1d50b9e4a55f878c25d134907d4458cf59
Merged-In: I0f0e937e56721d458e250d48ce62f80e3694900f