This is a fix for the fix in 319854d624. The original fix did
not reset the firstPeriodId to avoid any id clashes with future
updates. This however only works under the assumption that the
next manifest load at the next call to prepare() is exactly the
same as the current manifest. This is not true unless the call
happens very quickly (and may fail even then). Instead we should
keep the existing manifest directly as a reference so we can use
it to find the number of removed periods when we get a new manifest
at the next call to prepare().
Issue: androidx/media#1329
PiperOrigin-RevId: 634853524
b645004902