Use Base64 from android.util in LocalTransport

Stop using bouncycastle as requested in the bug.

Bug: 111440841
Test: 1. Without changes
         a. adb shell bmgr transport android/com.android.internal.backup.LocalTransport
         b. adb shell bmgr backupnow com.android.providers.settings
         c. adb shell ls /cache/backup/1/_delta/<kv_package> #=> Base64 encoded keys
      2. Build and flash this CL
         a. adb shell bmgr restore 1 com.android.providers.settings #=> verify stuff restored
         b. adb shell rm /cache/backup/1/_delta/com.android.providers.settings/* /data/backup/com.android.internal.backup.LocalTransport/com.android.providers.settings
	 c. adb shell bmgr backupnow com.android.providers.settings
	 d. adb shell ls
	 /cache/backup/1/_delta/com.android.providers.settings # Verify same keys as 1c

Change-Id: I305bbae0e0af3639c1d45def19872e6da84624df