fastbootd: Allow flashing the cache partition.

This fixes the following denial:
  avc:  denied  { write } for  pid=332 comm="fastbootd" name="mmcblk0p35" dev="tmpfs" ino=11234 scontext=u:r:fastbootd:s0 tcontext=u:object_r:cache_block_device:s0 tclass=blk_file permissive=0
  avc:  denied  { ioctl } for  pid=294 comm="fastbootd" path="/dev/block/mmcblk0p35" dev="tmpfs" ino=6953 ioctlcmd=0x1277 scontext=u:r:fastbootd:s0 tcontext=u:object_r:cache_block_device:s0 tclass=blk_file permissive=0

Bug: 150112538
Test: fastboot flash cache on non-A/B device
Test: fastboot erase cache on non-A/B device
Change-Id: Ib2288b42f2bb47e83b1476319669d6c2719db2ec
Merged-In: Ib2288b42f2bb47e83b1476319669d6c2719db2ec
1 file changed