DrmGrallocHelpers: make the DRM device path and name customizable

Originally, we hard-coded the DRM device path and name. However, the
value might be different between each platform. This CL makes them
customizeable by setting Android property.

Bug: 189059445
Test: android.media.cts.AdaptivePlaybackTest
Change-Id: If2418244b5854bafffd2d038dc85af323db7657f