Persist functions for oemOverride across reboot

persist OEM specific functions across boot using overlays when
ro.bootmode is NOT unknown
i.e. when phone boots up into a predefined Oem mode.

The overlay tuple has 4 columns instead of three where the fourth column
is optional. When the fourth column is present, the functions mentioned
there would be persisted across reboot along with adb(if enabled).
The fourth column is read during USB device manager set up
@readOemUsbOverrideConfig.

When trySetEnabled function is called, the override function is applied
and the actual oem functions are persisted in
persist.sys.usb.<bootmode>.config.
This property is used in an "on boot" property trigger to set up the
persistent function early in the boot.
(Similar to the way persist.sys.usb.config is used to setup the
USB functions during normal boot).
persist.sys.usb.<bootMode>.func tracks the functions without override.

For example, when the following tuple,

usbradio:adb:diag,serial_cdev,rmnet_gsi,adb:diag,serial_cdev,rmnet_gsi

when ro.bootmode is usbradio, and mCurrentFunctions is adb,
the actual functions enabled would be diag,serial_cdev,rmnet_gsi,adb
(sys.usb.config) and diag,serial_cdev,rmnet_gsi would be
persisted across reboots through persist.sys.usb.usbradio.config and
the functions would be saved in persist.sys.usb.usbradio.func

Bug: 31947358
Change-Id: Ifaef17f6943c1e70721cdc8489f17e3ece03bbfc