Remove mutator and use standard variant functions

As image variants are now supported directly by android.Module, this
removes a custom mutator in selinux_contexts and uses image variant
functions in android.Module.

InRecovery and InstallInRecovery may be confusing. But refactoring it is
out of scope for this CL.

Test: compare out/soong/build.ninja before and after
Change-Id: I9ebf665a1d50d24bb4e5568a4fd1af4c4eb02c90
1 file changed