com.android.mediadrm.signer is built with java_sdk_library

The com.android.mediadrm.signer is built with java_sdk_library
and api files are added by running "make update-api".
The com.android.mediadrm.signer is removed in platform.xml
since it will be made auto generation by soong when the library is
built with java_sdk_library.

Bug:77577799
Test: make -j
Test: make checkapi
Test: adb shell cmd package list libraries |\
      grep com.android.mediadrm.signer
      And check the com.android.mediadrm.signer library

Change-Id: I0e773615abe6aa8d757e489de2b9d807cd356146