Finer-grained locking for size operations.

Disk space measurements are read-only and don't perform mutations,
so other installd operations shouldn't block them.

If there's an ongoing parallel operation (such as a dexopt) that
could race and skew the results, that's no different than an actively
running app changing it's disk usage during the measurement.

This change also allows measurements to happen in parallel, so we can
no longer rely on getcwd() being stable, which means all fts(3) users
now need to use FTS_NOCHDIR.

Bug: 36032444, 35706513
Test: runtest -x frameworks/base/services/tests/servicestests/src/com/android/server/pm/InstallerTest.java
Change-Id: I67d303d3ecce148052d41444cef67381b1d34ab0