FileImage: make thread-safe

FileImage needs to be thread-safe because multiple
threads gets data from it when an incremental OTA
package is created.

Test: apply incremental OTA on cuttlefish
Bug: 113175337
Change-Id: I31637fce0fbd66f3fa6c5c478da09bae65a52229
Merged-In: I31637fce0fbd66f3fa6c5c478da09bae65a52229