ANDROID: GKI: power_supply: Add CP_ISNS_SLAVE power supply property

Add power supply property POWER_SUPPLY_PROP_CP_ISNS_SLAVE to
expose ISNS of slave SMB1390.

Signed-off-by: Sahil Chandna <chandna@codeaurora.org>
(cherry picked from commit 88073715)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 150789066
Change-Id: Iec34bf19070e2587a26a92abfefbea0da01fdd6b