Add Flag for Enabling 64-bit Media DRM Server

We are ready for devices to begin migrating to a 64-bit Media DRM
Server. However, not all devices are ready to make this jump yet. A
device needs to have all of its 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 mediadrmserver as 64-bit.

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

Test: Validated that turning on these flags enables a 64-bit
      mediadrmserver that loads 64-bit DRM Plugins from the lib64
      directory (after setting DISABLE_TREBLE_DRM to true to load this
      code as opposed to the Legacy DRM Plugin HAL)
Bug: 36076017
Change-Id: I141cfd57ff75b40c8b0cb40a5bf1439a1ab5b766