android: expand SAF FileOps implementation

[kari-ts]

This expands the SAF FileOps to implement the refactored FileOps

Updates tailscale/corp#29211

[nickkhyl]

(still reviewing)

[nickkhyl]

Other interfaces defined in libtailscale and implemented in Kotlin suggest that exceptions are supported will be translated to Go errors.

tailscale-android/libtailscale/interfaces.go

Lines 59 to 60 in b3626fc

	// GetSyspolicyStringValue returns the current string value for the given system policy.
	GetSyspolicyStringValue(key string) (string, error)

tailscale-android/android/src/main/java/com/tailscale/ipn/App.kt

Lines 347 to 355 in b3626fc

	@Throws(
	IOException::class, GeneralSecurityException::class, MDMSettings.NoSuchKeyException::class)
	override fun getSyspolicyStringValue(key: String): String {
	val setting = MDMSettings.allSettingsByKey[key]?.flow?.value
	if (setting?.isSet != true) {
	throw MDMSettings.NoSuchKeyException()
	}
	return setting.value?.toString() ?: ""
	}

If that's correct, is there a reason we chose to report errors by returning nil values here and in other ShareFileHelper methods?

[nickkhyl]

LGTM with the androidFileInfo marshalling fixed and other feedback considered.

[nickkhyl]

Close wc if GetFileURI fails?