Clear cached files on external storage.

When clearing cached files belonging to an app, include any cached
files on external storage.  Since we need to keep sdcardfs in the
loop about any file deletions, we always mutate by going through the
sdcardfs layer instead of behind its back.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 37486230, 37566983, 37913442, 37914374
Change-Id: If174bf7eaf682da83cf0ab1b4938fe9a5956d464