Consolidate the vendor space misc usage for Pixels

The layout of the vendor space /misc partition was pretty confusing and
lead to some usage conflicts. To formalize the layout, we create a pixel
specific library with the definition & offset of various flags. The new
library also handles the R/W. As a result, we will leave system domain
/misc definitions in the libbootloader_message.

We also switch the misc_writer binary to use more specific options
instead of writing an arbitrary hex string. So we can avoid redefining
the string & offset in both init script and recovery ui.

Bug: 131775112
Test: unit tests pass, run misc_writer and check contents of /misc
Change-Id: I00f8842a81d1929e31a1de4d5eb09575ffad47c0