Make installed-files-recovery.txt depend on rsync timestamp

Files are copied into the recovery installation directory by a rule
that runs rsync.  The previous logic tried to avoid racing with the
rsync rule by depending on the final packaged recovery image, but
if BOARD_MOVE_RECOVERY_RESOURCES_TO_VENDOR_BOOT is set then
INSTALLED_RECOVERYIMAGE_TARGET is left empty and no dependency was
added.  Depend on the timestamp file used as the output of the rule
that runs rsync instead.

Test: query dependencies of installed-files-recovery.txt
Fixes: 173772240
Change-Id: I5ec77a6933e3a83fe4e4819e0e246017d824d8a5