Manifest flag for apps to request sandboxing.

We're iterating on the design to let apps request via a manifest
flag if they'd like to opt-in or opt-out of storage sandboxing.  The
default is to assume that P apps don't allow sandboxing, and that
Q apps do allow sandboxing.

Bug: 129487770
Test: manual
Change-Id: I8fddb35eed412c78622be4ccae57c7b7d2aa1286