Update shared sandbox prefix to "shared-".

":" is not allowed in filenames by vfat.

Bug: 119525489
Test: sm set-virtual-disk true
Test: atest core/tests/packagemanagertests/src/android/content/pm/KernelPackageMappingTests.java
Test: atest services/tests/servicestests/src/com/android/server/StorageManagerServiceTest.java

Change-Id: I95ef58486abe8f2a9760e0229192448e0bd9ef42