Apply OTA from /vendor/boot_otas

If BOARD_16K_OTA_MOVE_VENDOR := true is set, OTAs will be present at
/vendor/boot_otas for 16KB developer option. Apply OTA from vendor if
present, otherwise fallback to system OTAs

Test: m, m Settings && adb install -r $ANDROID_PRODUCT_OUT/system_ext/priv-app/Settings/Settings.apk
Bug: 335022191
Change-Id: I534b354c830de2c90fee76502667d5e4ef5a7714