app/internal/log: use the app ID as the Android log tag

This way, a Gio app's logs can be filtered uniquely, which wasn't
possible before since the tag "gio" would be the same between gio apps.

Since the app ID is supplied at build time, inject it via a variable
with the linker's help. The variable is only used on Android for now,
but that's OK. It might be useful for other platforms or other internal
packages in the future.

Fixes #84.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

app/internal/log/log_android.go

@@ -22,7 +22,7 @@ import (
 // 1024 is the truncation limit from android/log.h, plus a \n.
 const logLineLimit = 1024
 
-var logTag = C.CString("gio")
+var logTag = C.CString(appID)
 
 func init() {
 	// Android's logcat already includes timestamps.