Move TranslatingCursor to frameworks/base.

so that it can be used by DownloadProvider as well.

Also, override getColumnNames() as well since this is
what is effectively used for getting the column count.

Bug: 111890351
Test: atest MediaProviderTests
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: I336729c321614d923a31521379896ce1b88ad6b0