Describe the bug
FHIR Engine search results sometimes returns outdated results because old indexes are not deleted

Component
Core library

To Reproduce
Steps to reproduce the behavior:

1. Select a resource to test with by it's ID and resourceUuid from the database
2. Write code to fetch and call fhirEngine.update on the same resource
3. Check the DateTimeIndexEntity table using Android Studio app inspection for the _lastUpdated indexes for the specific resource
4. You'll realize that there are multiple indexes for the same property

Expected behavior
A clear and concise description of what you expected to happen.

Inspecting the database should show only a single record for the resource's _lastUpdated index. This should be the latest index

Screenshots
N/A

Smartphone (please complete the following information):

• Device: Nexus 5X API 28 emulator
• Android version: 9
• Build number: PSR1.180720.117
• Version 28

Additional context
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