Avoid log spam for the common "no user keys" case.

We see this a lot in logs, and it causes a lot of people to hassle us
about what is actually a common case:
```
 AdbDebuggingManager: Cannot read user keys
 AdbDebuggingManager: java.io.FileNotFoundException: /data/misc/adb/adb_keys: open failed: ENOENT (No such file or directory)
 AdbDebuggingManager: 	at libcore.io.IoBridge.open(IoBridge.java:574)
 AdbDebuggingManager: 	at java.io.FileInputStream.<init>(FileInputStream.java:160)
 AdbDebuggingManager: 	at android.os.FileUtils.readTextFile(FileUtils.java:638)
 AdbDebuggingManager: 	at com.android.server.adb.AdbDebuggingManager.dump(AdbDebuggingManager.java:1801)
 AdbDebuggingManager: 	at com.android.server.adb.AdbService.dump(AdbService.java:604)
 AdbDebuggingManager: 	at android.os.Binder.doDump(Binder.java:1030)
 AdbDebuggingManager: 	at android.os.Binder.dump(Binder.java:1020)
 AdbDebuggingManager: 	at android.os.Binder.onTransact(Binder.java:890)
 AdbDebuggingManager: 	at android.debug.IAdbManager$Stub.onTransact(IAdbManager.java:357)
 AdbDebuggingManager: 	at android.os.Binder.execTransactInternal(Binder.java:1362)
 AdbDebuggingManager: 	at android.os.Binder.execTransact(Binder.java:1301)
 AdbDebuggingManager: Caused by: android.system.ErrnoException: open failed: ENOENT (No such file or directory)
 AdbDebuggingManager: 	at libcore.io.Linux.open(Native Method)
 AdbDebuggingManager: 	at libcore.io.ForwardingOs.open(ForwardingOs.java:563)
 AdbDebuggingManager: 	at libcore.io.BlockGuardOs.open(BlockGuardOs.java:274)
 AdbDebuggingManager: 	at libcore.io.IoBridge.open(IoBridge.java:560)
```

Test: treehugger
Change-Id: I5206f8dada0d97c022459c2370b839c209041539