Don't hard-code number of tombstones.

On cuttlefish, the number of tombstones allowed is much larger
than 50, so change the algorithm to search for any tombstone
file.

Test: Ran unit tests on cuttlefish with > 50 tombstones.
Test: Ran unit tests on device.
Change-Id: Ia1d885fe19a7f7751fe3386d40b48750d1e21bd5