[Feature][task-view-765][FOTA]add api for get current slot

Only Configure: No
Affected branch: ZK-first-oem-release
Affected module: FOTA
Is it affected on both ZXIC and MTK: only MTK
Self-test: Yes
Doc Update: No

Change-Id: I7ed38c9d5f16f14b255f3e7a6b6062b25e932dd8
2 files changed