Remove javaMockFS()

Also, removes the workaround that allowed preparers to override files
added in javaMockFS().

The performance of the java tests was determined by running:
  m nothing
  rm -fr out/soong/.bootstrap/soong-java/test/
  time m nothing

Prior to starting this work that returned:
  real  1m30.060s
  user  33m3.028s
  sys   13m25.413s

Immediately prior to this change that returned:
  real  1m2.777s
  user  14m14.352s
  sys   9m40.745s

After applying this change that returned:
  real  0m20.282s
  user  3m7.429s
  sys   0m43.601s

So, this reduced the real time cost of running the java tests from 90
seconds to 20 seconds.

Bug: 182638834
Test: m nothing
Change-Id: I30490fd40ddb0684a5a4975ce1811bab5dc79e4e