Freezer: release AM lock while checking /proc/locks

Checking /proc/locks can take a long time if new file locks are being
added. Hoist that outside of the AM lock to make sure that
system_server doesn't stall unnecessarily.

Test: device functions with short freezer timeout
Bug: 269207437
Change-Id: I5bab77281992d0e826427e458025103500fc0869
Merged-in: I5bab77281992d0e826427e458025103500fc0869