Check sepolicy context for mounting vendor overlay

The vendor overlay source directory must have the same sepolicy
context with the target vendor directory.
Otherwise, the vendor overlay will fail to mount.

Bug: 112955896
Test: Check if a source directory that has a differnt context fails
      to mount on vendor directory.
Change-Id: I2522a8ddc96d4d9b0f34d25a081d20d1ac2dd27a