Fix several issues in framebuffer_service

-supports fb's with x and y offsets
-dumps the front buffer (instead of just the low end of the buffer)
-switch from mmap to read/write, for fb's with very large offsets

Change-Id: Id560790c9661f7cc3b4350a44cc29f0104831e85
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>