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.
Util.toUpperInvariant() is null-tolerant, while Ascii.toLowercase() is
not. Most usages in this change are clearly non-null. The BandwidthMeter
usages aren't annotated @Nullable, but the current code *would* work if
countryCode was null in both cases. These methods will now throw NPE if
they're passed null.
PiperOrigin-RevId: 368816287
