Enable cache clearing v2 logic, fix bugs.

Now that we have CTS tests to verify the cache clearing v2 logic,
we're ready to enable it!

Switch storage APIs over to "usable space" to match implementation
down in installd.  Add a missing clearCallingIdentity(), and offer
better logging for missing appop permission.

Load quotas from disk right away at device boot instead of waiting.

Bug: 35685848, 35684969
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Change-Id: I8167e0322b4662ca6c975da5c50715e2d71332a7