Remove null support in SharedFileRegion

ParcelFileDescriptor does not support null (negative) file descriptors
so we remove it from SharedFileRegion and expect that a @nullable
SharedFileRegion is used for cases where a null file descriptor is
allowed.

Test: atest shmemTest
Change-Id: Icf226181cf6de9759ed3145ff6b5ad0d8fd650af
7 files changed