Flattened APEXes are activated

apexd now can activate flattened APEXes. Specifically,
/system/apex/<name> directory is bind-mounted to
/apex/<package_name>@<version> directory, which is in turn again
bind-mounted to /apex/<package_name> if the <version> is the latest.

Bug: 118485880
Test:  TARGET_FLATTEN_APEX=true m apex.test;  TARGET_FLATTEN_APEX=true m
/apex/com.android.example.apex exists.
Change-Id: I0a1087bb37d9ad6af5dd99539e13e2fea5181336
6 files changed
tree: 1d48bcd093d7eac0d9c1bbad0279e8a3b395f6bf
  1. apexd/
  2. apexer/
  3. testdata/
  4. Android.bp
  5. OWNERS
  6. PREUPLOAD.cfg