Add PM checker for AppOpsManager

Added null == pm to deal with the possible null pointer problem when
LowRamDevice gets AppOps

Bug:230592908
Fixes:237613199
Test: run gts -m GtsPreserveLegacyStorageHostTest -t android.scopedstorage.cts.host.PreserveLegacyStorageHostTest#testPreserveLegacy

Change-Id: I80f1662a694886ef2acde58ae3eec50254e08aa2