Soong: Add mk2bp conversion for prebuilt_usr_share.

There are a couple of makefiles that have etc with $(TARGET_OUT)/usr/share
as the local module path. Added the conversion in androidmk for target
and host.

Bug: b/132123818
Test: Wrote and ran unit test cases, did a test conversion on
      external/neven/Android.mk makefile.

Change-Id: Iafed89f4cee499f561a1235f8870b1f1329e99bc