Fix potential race in DexManager

The internal cache of DexManager holding information about the code
locations may race if an install happens at the same time with a dex
load.

Test: runtest -x services/tests/servicestests/src/com/android/server/pm/dex/DexManagerTests.java
Bug: 37922353

Change-Id: Ia28fb54d6859ec185aaaf8e0ed68981004b173e5