Add ability to show "installing security update"

Add a new command "--security" to boot commands. If this command is
observed as part of BCB, choose a different background text picture
for installing stage in recovery UI. As a result, users will see
"installing security update" instead of "installing system update"
when applying a security update package.

Bug: 27837319
Change-Id: I2e2253a124993ecc24804fa1ee0b918ac96837c5