Introduce postMount() VolumeBase helper.

Introduce postMount() VolumeBase helper.

When we're mounting a private volume, we create stacked emulated volumes
on top of it. Due to the ordering there, we would broadcast the emulated
volumes being created *before* the "mounted" status update. This in turn
could cause us to try and mount these emulated volumes before the
underlying private volume is really mounted. This is problematic in
particular on devices that support a filesystem keyring, where we need
to do some additional setup before the devices can be used.

While we could modify StorageManagerService to delay the mount, a safer
fix at this stage of the release is to just fix the ordering of these
events. To achieve that, add a simple postMount() helper, that is called
after a succesful mount. This allows us to setup the volume properly
before trying to mount any stacked volumes.

Bug: 151079464
Test: atest AdoptableHostTest
Change-Id: I2cc4113d4d71d89aa629bb9c0fa9be441355c079