Make Cover Art storage more robust to filesystem issues

We've being seeing conditions where external application storage fails
to come up. While that is a separate issue that needs to be fixed, the
way our storage handles those errors is our responsibility. Currently,
some things are crashing. This patch makes it so all file operations
check the path before proceeding. It also makes us be more aggressive in
clearing storage on stop and start so we can always try to start in a
good state if storage misbehaved previously.

Bug: b/151478590
Test: atest BluetoothInstrumentationTests
Change-Id: Iae0f29ef4b75d2c001642685c4a4b76da43bb7ed
Merged-In: Iae0f29ef4b75d2c001642685c4a4b76da43bb7ed