Look for visible vols for a user under /mnt/user/<userId>/package/.

Currently, we pass down visisble volumes for a user to zygote so that
it can set up appropriate bind mounts after forking. This method has
atleast a couple of race conditions, so instead make zygote just
look under /mnt/user/<userId>/package for volumes that need to be
handled for a particular user.

Bug: 126069494
Test: manual
Test: atest cts/hostsidetests/appsecurity/src/android/appsecurity/cts/ExternalStorageHostTest.java
Test: atest cts/tests/app/src/android/app/cts/DownloadManagerTest.java
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: Iab47eb34d38a25dfda4a686db9b7203ad4885a93