Use temp mounting when running pre/post-install hooks

* apexd will temp-mount all the apexes in the session and then fork into
  apexd --pre-install <mount-point-with-hook> [<all-other-mount-points>]
* Now during submitStagedSession temp-mounting happens twice:
  in verifyPackages() and in preInstallPackages() stages. This will be
  fix when we will keep temp-mount alive while apexd session is not
  verified/canceled.
* Since I was there, added some comments in pre/post-install flow
  handling to make it easier for future readers to follow the code.

Note that adb install com.android.art.debug.apex still fails during
preInstallPackages(), but this failure is most likely specific to art
and doesn't come from apexd.

Test: adb install ${OUT}/system/apex/com.android.art.debug.apex
Test: verified that install doesn't fail with mount point clash issue
Test: adb shell cmd apexservice preinstallPackages /system/apex/com.android.art.debug.apex
Bug: 140935513
Change-Id: Ia67d3af60693ff485e17915ebb5a140a81d36542
9 files changed