Migrate CTS to separate broadcast to scan volume.

It can't sent MEDIA_MOUNTED via the shell, so let's use a separate
broadcast.  Also improve robustness of several tests.

Bug: 123251175
Test: atest cts/tests/tests/media/src/android/media/cts/MediaScanner*
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: I5b0f07c817b18175c6e427729a9fa0545f6bcd8e