Let vold execute mke2fs.

When adopting SD cards, vold partitions and formats those devices;
this had been working fine with the older make_ext4fs utility, but
newer devices are switching over to mke2fs, which has a different
SELinux label.

avc: denied { execute } for name="mke2fs" dev="dm-0" ino=456 scontext=u:r:vold:s0 tcontext=u:object_r:e2fs_exec:s0 tclass=file permissive=1
avc: denied { read open } for path="/system/bin/mke2fs" dev="dm-0" ino=456 scontext=u:r:vold:s0 tcontext=u:object_r:e2fs_exec:s0 tclass=file permissive=1
avc: denied { execute_no_trans } for path="/system/bin/mke2fs" dev="dm-0" ino=456 scontext=u:r:vold:s0 tcontext=u:object_r:e2fs_exec:s0 tclass=file permissive=1
avc: denied { getattr } for path="/system/bin/mke2fs" dev="dm-0" ino=456 scontext=u:r:vold:s0 tcontext=u:object_r:e2fs_exec:s0 tclass=file permissive=1

Test: cts-tradefed run commandAndExit cts-dev --abi armeabi-v7a -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.AdoptableHostTest
Bug: 36757864, 37436961
Change-Id: Ifb96dfca076ea58650eb32f89e850f20ae2ac102