Export TARGET_ENABLE_MEDIADRM_64 to Soong.

The change enables conversion of
hardware/interfaces/drm/1.0/default from make to Soong
by exporting a variable used in a conditional check by
the modules in that directory.

Bug: 161571531
Bug: 123557988
Test: Build with TARGET_ENABLE_MEDIADRM_64 :=
Test: Build with TARGET_ENABLE_MEDIADRM_64 := true
Test: Insepction of out/soong/soong.variables for each
Test: Inspection of target files package for each
Test: Verify inclusion of modules in vendor snapshot
Change-Id: I5737d9d4687e182595a213be000b2e56c7536b8a