libhwc2on1adapter: Move to vendor partition

libhwc2on1adapter is a shim layer for legacy hwcomposer HALs.  This is
only used by android.hardware.graphics.composer@2.1-impl.so.  This must
not be accessed by framework modules.  Thus, this commit moves
libhwc2on1adapter to the vendor partition and removes vendor_available.

Bug: 38302528
Test: Sailfish builds and boots.
Change-Id: I43d66c8c62f3dc90e75805f6a459573c093f9f14