Even when fixed to the US locale (and thus avoiding surprising behaviour
in e.g. Turkish locale with "i" and "I") there are unexpected behaviours
when upper and lower casing non-ASCII characters.
For example it's sometimes not symmetric, e.g.:
"ẞ".toLowerCase() -> "ß"
"ß".toUpperCase() -> "SS"
In all the ExoPlayer usages we are either dealing with known-ASCII
strings (e.g. MIME types) or comparing against ASCII constant strings
anyway, so it seems easier to just use Guava's ASCII-only class in these
cases.
This change also includes some null-twiddling, because
Util.toLowerInvariant() is null tolerant, while Ascii.toLowerCase() is
not. Most of the usages were already non-null, and it was easy enough to
change the remaining ones to be so by simple reordering of statements.
I'll make an equivalent change for Util.toUpperInvariant() next.
PiperOrigin-RevId: 368419813
