Return installer mount mode for installers.

In the new FUSE world, we're interested to know the difference between
installers and apps that previously asked for WRITE_EXTERNAL. For
devices without FUSE enabled, they both map to the "write" sdcardfs
view, so for them this is a no-op.

Bug: 146189163
Test: device boots with and without FUSE
Change-Id: I634829eec8f0e7e65e4e740ce216dded23abc271