Test: Upgrade to Junit4 in Java instrumentation tests

* AndroidTestCase base class is deprecated, this CL upgrades all
  instances of it to Junit4 based Java instrumentation test
* Also moves FileSystemWriteTest to com/android/bluetooth as original
  core module does not exist

Bug: 68015458
Test: runtest -j40 bluetooth
Change-Id: I223cb748ee908b31aafc9b1b61b366ed23b2f990