adb: set main thread in transport tests.

Previously, we were relying on tests run previously to set the main
thread ID, which fails when the tests are sharded.

Test: for i in `seq 0 4`; do GTEST_SHARD_INDEX=$i GTEST_TOTAL_SHARDS=5 /data/nativetest64/adbd_test/adbd_test; done
Change-Id: I15ce70b4598b6ffcf3f5467d16c917f23f668daf