fastbootd: Only flash slots listed by the boot control HAL.

Bug: N/A
Test: flash when metadata slot count is >2
Change-Id: I67481be0de162cab5da8d32c2e318489427f1932
Merged-In: I67481be0de162cab5da8d32c2e318489427f1932
(cherry picked from commit 8568dcb057d63023feca09b031e456592c133f0e)
4 files changed