This field was not being set but was used in storage_manager.cpp:89,
which constructed a String16 using the nullptr. This internally
calls strlen on the null pointer, which causes a seg fault. Since
mFileName is not being set and getFileName has no other callsites,
remove the field altogether so nobody else tries to use it and pass
in the expected value at its only callsite.

Test: Calling AStorageManager_mountObb from NDK does not result in
a crash.

Signed-off-by: Sam Gao <ssgao@meta.com>

Upstream from Meta.

Change-Id: I4374148f466dacd62c659d3c94fd309674cb2d34