libdrmclearkeyplugin is linked against libstagefright_foundation_vendor

Since libdrmclearkeyplugin is in vendor partition, it is linked against
the vendor-variant of the libstagefright_foundation.

Bug: 37343126
Test: sailfish builds and boots
Test: BOARD_VNDK_CURRENT=current m libdrmclearkeyplugin is successful
Change-Id: Id8302f88978fc91282c54e0a679bcd8978898f8d