Speed up BackgroundDexOptimizationTest.

This change removes the need to reboot and wait for post boot
optimization to complete in test cases for idle optimization, which
gives the test more chance to finish within the time limit.

Also, this change migrates BackgroundDexOptimizationTest to using
BaseHostJUnit4Test.

Bug: 220924072
Test: CtsCompilationTestCases:BackgroundDexOptimizationTest (now takes
  ~11mins ab/I81800010023201694)
Change-Id: I85b14eb1d460da7f365b0c11e8e800d3d5fce751
Merged-In: I85b14eb1d460da7f365b0c11e8e800d3d5fce751
(cherry picked from commit 4ba16902be7cdad6c6615d01725cb407fae8b824)
1 file changed