Service for reading and writing blocks to PST partition

Permits apps with permission
android.permission.ACCESS_PERSISTENT_PARTITION to obtain
a read and write data blocks to the PST partition.

Only one block ever exists at one time in PST. When
a client writes another block, the previous one is
overwritten.

This permits storing a block of data that will live
across factory resets.

Change-Id: I8f23df3531f3c0512118eb4b7530eff8a8e81c83