Single character noun user dictionary entry may not work

[yukawa]

Environment

• Android M 6.0.1 MTC20F (not specific to a certain Android OS version)
• Nexus 6P (not specific to a certain device)
• Mozc 2.18.2556.103 (ae6bbac)

Steps to reproduce

1. Install Mozc for Android
2. Open user dictionary tool
3. Add a new user dictionary entry
   • Word: test1
   • Reading: か
   • Category: Noun
4. Add a new user dictionary entry
   • Word: test2
   • Reading: し
   • Category: Noun
5. Focus on an EditText.
6. Type か
7. Hit backspace key
8. Type し

Expected behavior

• After the step 5, test1 is included in the candidate list.
• After the step 7, test2 is included in the candidate list.

Actual behavior

• After the step 5, test1 is not included in the candidate list.
• After the step 7, test2 is not included in the candidate list.