[incfs] Correctly wait for async .so's extraction

StorageID for an installation changes as we go from a staging
directory to the final one. That's why the only correct way
of waiting for the extraction to complete is by the whole
MountID, even if later it would cause the call to wait on all
instances of the app.

+ measure the waiting timing

Bug: 153513507
Test: adb install megacity.apk
Change-Id: I83558f155867ae5503719ecb63d591fc969c3995