Removes static variables in backed_block.c to allow multiple
sparse files to be open at the same time.
Change-Id: I012d8a424c6e21a7352408416adb7c72ee8add21
This moves an exact copy of libsparse from
system/extras/ext4_utils/libsparse to system/core/libsparse in
preparation for linking tools in system/core against it.
Change-Id: If664e4fcfd6612844ac745589beb1517e7f9fe58