Add Flag for Enabling 64-bit Legacy DRM Plugins

We are ready for devices to begin migrating to a 64-bit Legacy DRM
Loader. However, not all devices are ready to make this jump yet. A
device needs to have all of its Legacy DRM Plugins ready to run as
64-bit before it can do this. This includes having 64-bit hardware
integrations.

So that devices can turn on 64-bit support as they get ready, I am
adding flags that can be set in a device's device.mk file. There are two
parts:

* ENABLE_MEDIADRM_64 enables building the Legacy DRM Plugin HAL as
  64-bit.

* Setting drm.64bit.enabled=true in PRODUCT_PROPERTY_OVERRIDES switches
  the Legacy DRM Plugin HAL to searching for DRM Plugins in the 64-bit
  directory.

Test: Validated that turning on these flags enables a 64-bit Legacy DRM
      Plugin HAL that loads 64-bit Legacy DRM Plugins from the lib64
      directory.
Bug: 36076017
Change-Id: I8d21fc7e7c0357eb6755839d41caa0f37ca20aff