Track MimeMap moving to libcore.content.type.

Another CL in this topic moves libcore.net.MimeMap to package
libcore.content.type. This CL:
 - removes an unused import that has internally been removed by commit
   b6781bc1e43beb3c88ffa95530d6a96935fa0f48
 - Changes one use of libcore internals to a public API, which is a
   part of what internal commit ad8ef4ef3ea75f61224d1786d12cd9257aca1ecf
   also did.

Hopefully this will avoid merge conflicts, because the changes have
already been made downstream.

Bug: 136256059
Bug: 124232146
Test: Treehugger
Test: CtsMimeMapTestCases
Exempt-From-Owner-Approval: move affecting imports only
Change-Id: I0430c7840466d07d93bf6e16810bf40bc330d1cc
2 files changed