Remove dependency to libmediadrm from drm HAL

drm HAL has been using libmediadrm for android::SharedLibrary class and
android::PluginLoader class. This is a Treble violation since
libmediadrm is a framework-only library that is not available to
vendors.

To solve the problem, the two classes are copied into this directory
to form a small static library android.hardware.drm@1.0-helper.a.

Bug: 38302533
Test: mm under /hardware/interfaces/drm/1.0/default and
/hardware/interfaces/drm/1.0/vts/functional

Change-Id: I7b4e5cdb3bc815e971e0c3b7ec99ea86042e13eb