Handle screendumps for all framebuffer sizes

readx() treats a partial read as an error but also consumes the data,
Now exactly the amount of data needed for the screendump is requested.

This bug showed up for framebuffers that were not a multiple of 640
bytes.

Also fix a compiler warning related to handing pipe() failure.

Change-Id: I8b1713923e156d4e3424784152e5dc5cbc7d478d
Set security context of /adb_keys and /data/misc/adb/adb_keys.

I97b3d86a69681330bba549491a2fb39df6cf20ef introduced a separate type
for the adb_keys file.  Set the security context of the adb_keys file
accordingly by adding restorecon commands to init.rc.

Change-Id: I30e4d2a1ae223a03eadee58a883c79932fff59fe
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>