fastbootd: Unmount /cache before flashing.

Bug: 150112538
Test: fastboot flash cache on non-A/B device
Change-Id: I8a86bc4d12e4b3be020bbe47e02262a5aaa913a7
Merged-In: I8a86bc4d12e4b3be020bbe47e02262a5aaa913a7
2 files changed