Suppress denials from idmap reading installd's files.

We are occasionally seeing the following SELinux denial:

avc: denied { read } for comm="idmap" path="/proc/947/mounts" scontext=u:r:idmap:s0 tcontext=u:r:installd:s0 tclass=file

This commit suppresses that exact denial.

We believe this is occurring when idmap is forked from installd, which is reading its mounts file in another thread.

Bug: 72444813
Test: Boot Walleye and test wifi and camera.
Change-Id: I3440e4b00c7e5a708b562a93b304aa726b6a3ab9