[pm] fix thread tag

PermissionManagerServiceImpl also uses the "PackageManager tag". As a
result, we might see two "PackageManager" threads in the threads dump
which is confusing. Fixing the tag to be "PermissionManager".

BUG: 245494249
Test: builds
Change-Id: I751b737dd5d5c6ab246012c9c09c9bb990fcc912