perf(r8): keep exception names without keeping every exception

[bmc08gt]

Replaces #1372, which GitHub auto-closed when #1371 merged and its base branch was deleted. Same change, rebased onto code/cash.

-keep public class * extends java.lang.Throwable kept 972 classes whole. Only their names are load-bearing.

Why the names have to stay

The Coinbase onramp traces send it::class.simpleName as errorType (CoinbaseOnRampController.kt:326,385,451), and Events.kt:90,103 does the same for analytics. By the time those leave the device they are plain strings, so the Bugsnag mapping upload cannot repair them. Deleting the rule outright would turn every errorType in telemetry into a one-character name.

Why the members and the classes themselves need not

Nothing reads the members, and an exception nothing constructs need not survive. -keepnames is the rule that states exactly that.

Measured, release build

	-keep	-keepnames
DEX (uncompressed)	16,101,032	16,028,404
classes	21,361	21,020
members	1,330,404	1,329,949

72,628 bytes, 341 classes.

The dropped set is what the old rule was propping up — 316 of 357 have Error or Exception in the name, led by com.flipcash.services.models (94) and com.getcode.opencode.model (41). AddReactionError and its six subclasses are representative: ChatMessagingService.addReaction has no caller outside tests, and R8 had already shrunk the method in both builds, so the rule was holding the error classes alive on their own.

CoinbaseOnRampApiError and GetJwtError subclass names are unchanged in the new mapping.

Two comment corrections

Both rules were described as doing something other than what the code does.

The AppRoute rule is not for analytics. annotatedEntry derives each screen's root test tag from the route's simple name via NavMetadata.screenRootTag, so obfuscating those renames every screen-root resource-id the UI tests address. The rule stays; the comment now says why.

The com.kik.scan wildcard is genuinely load-bearing. The scanner's JNI constructs those classes by name from C++ (FindClass("com/kik/scan/UsernameKikCode"), GetMethodID for <init>) and reads their backing fields through GetFieldID (_username, nativePtr) — none of which AGP's default -keepclasseswithmembernames class * { native <methods>; } covers. Five classes, so the wildcard costs little.