Both build 32 and 64 architecture for TetheringTests

Compile as multilib is neccessary, otherwise mts-tethering-coverage
test plan will fail on armeabi-v7a because of failing to extract
native libraries. This change also add mts tag to TetheringTests.

Bug: 145490751
Test: -atest TetheringTests
      -m mts && mts-tradefed run mts-tethering-coverage

Change-Id: I6425c135ca17213bcdd5198177993daf8280504e